looked like she'd seen a ghost. >> it came as a shock to everybody. >> it was probably the hardest moment of my life. >> their mom in a new rue mans after a painful divorce killed. a tragic fall in a turbulent river. her new husband in a desperate race to save her. >> he was very distraught.

>> saying we needed to call 911. >> it wasn't until the funeral was over that the suspicions emerged. >> she said, you shoved her off that cliff, didn't you? >> was this an accident or was it a crime? >> you're pursuing a theory of a suspicious death? >> correct. >> police lad a haunch, but where was the evidence? >> we don't just roll the dice to see if something will stick. >> near an old western ghost town, the question haunts. >> if she was your mother, would you not try everything? >> what really happened along that raging river? >> what really happened along that raging river? the mystery at chalk creek. captions paid for by nbc-universal television it was a story that seemed to tell itself. the body found in a place where hikers have fattal accidents. but was this fall really an accident? what began as a fishing trip would later become a hunt for the truth of what happened that day. here's dennis murphy.

>> reporter: chalk creek thunders out of the rockies in the late spring. it is snow melt, furious whitewater, as it sluices its way down the canyon, by the old colorado ghost town. just imagine losing your footing here and tumbling headlong. the story they'd tell later would make you flinch when you actually saw the spot. the place where the mother of three left this life. >> i said, "well, mom's dead." >> but then it's still all so mysterious what actually happened up here on chalk creek that day. what happened to nancy mason. >> you know, what if it wasn't an accident? >> i would describe her as caring and loving. >> always upbeat and happy. >> she loved to entertain and she loved to be with family. >> reporter: there are three

things that everyone who knew her will tell you about nancy mason. she liked parties, was born to take care of others and she loved to read. nancy's parents bill and mariam. >> she always had a book. she read every nancy book they had available. nurse nancy, nancy drew. >> reporter: but it was nurse nancy, not detective nancy, that would inspire her career path. she became a neonatal nurse on the night shift watching over the preemies. so nancy. it was an american life like so many others. she'd married her first boyfriend, a kid she met at church camp, and together thy had three sons. wesley is the baby. >> i was the youngest of three boys. i was -- i was kinda spoiled. >> reporter: the family was just nuts for hockey. and spent many a weekend in denver's glorious, mountain backyard. but nancy? for her, sports were something you happily watched from the stands or a comfortable chair. >> mom would go with us to go fishing, but she had a lawn chair and a book.

how close is the parking lot to the lake? >> reporter: nancy was the kind of mom who kept meticulous scrapbooks. there are all the holidays and the fun vacations. but you won't find the event recorded there that blew a hole in their family in 2002. that's when nancy's husband todd, a wealthy probate attorney, up and left after 26 years. their oldest son, matt. >> it was horrible. it was all my dad's decision. he was done with that part of his life. >> usually upbeat, nancy was, by all accounts, slipping into a deep depression. >> she would just cry all the time. >> reporter: nancy, the caretaker, desperately needed someone to look out for her a little. so she joined a divorce recovery group at her church. and slowly the fogbank of her sadness began to lift. it was there she met a man who just maybe could dry her tears. his name was dan mason. >> she would say things like,

"he's always in the middle of all the fun. he's the life of the party." >> reporter: did she need that? >> yes, and he was there when she cried. he was there to pat her and -- and tell her that everything was okay. she wasn't alone. >> reporter: the ink was barely dry on nancy's divorce papers when dan, the life of the party, proposed a vegas wedding. >> the nervous groom tying the tie. >> reporter: it was all a little quick to be sure, but nancy's parents and son matt made an effort to be supportive. >> i was happy for her. anything to make her happy. i encraged it. >> reporter: but son wes, not so much. 16 at the time, he didn't care for new dad. >> you know when you walk into a car dealership and that guy walks up to you, and just starts talking, talking, talking? >> reporter: that person is dan? >> that was dan. >> reporter: trying too hard? >> trying way too hard. >> reporter: wes went out of his way to avoid his mom's new husband. and the new life they were building together. a new start financed by nancy's

handsome divorce settlement. and the new husband came with a package deal. along with him, nancy was getting his young friend efren gallegos, too. a kind of sidekick of d's. a guy who dan, and now nancy, were ting under their wings. he'd work with dan on some siness ventures and help nancy around the house. >> and, of course, nancy looks after everybody. and they made room for him to move in. >> reporter: and six months into the new marriage, dan, nancy and their friend efren decided to take a fishing trip over memorial day weekend. dan was familiar with the canyons around salida, colorado, about three hours from denver. so off they went. and even though her family says nancy had never been one for lacing up the hiking boots -- far from it -- dan would later recall that on sunday morning, the three took a hike on the old gold-miners trails. out past the ghost town of st. elmo. later, they stopped at a fishing spot on chalk creek just below the ghost town. it was secluded. how the river roared.

coming up -- >> it was -- probably the hardest moment of my life. >> reporter: danger in the water. >> dan was just saying that we need to call>> reporter: an. >> reporter: and a race to save a life.po when the mystery at chalk creek continues. - the budget masters. - ♪ yeah yeah the knock-out artists who are finding more ways to spread their dollar further. - to bolder color in less time. - ♪ are you feeling it? say hello to newer ideas and lowered prices, enabling more people to turn more saving into more doing. - that's the power of the home depot. - ♪ are you feeling it? try out different colors with new 8-ounce paint samples at a new low price of only $2.94. when people say, "hey mike, why ford? why now? you know what i do? i introduce them to the most fuel-efficient midsize sedans... and suvs in america. i don't know if you've heard, but this whole fuel-efficiency thing... kind of a big deal. anyway, ford and lincoln mercury have you covered.

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eat and hike and take our atv out. >> reporter: judy and lynn cleveland were kicking back that memorial sunday afternoon. taking the atv for a spin on the dirt road between their family's weekend cabin and the old ghost town. but -- all of a sudden here came two men in an suv flagging them down. they would turn out to be efren gallegos and nancy's new

husband dan mason. >> he was very distraught at the time. >> dan was just saying that, "my wife has been hurt. she's fallen in the creek and we need to call 911." >> reporter: cell phones didn't work in the canyon so they led dan to their cabin so he could call 911 on the landline. with sheriff's deputies alerted, the couple grabbed some blankets and followed the men in the suv back to where the driver said his wife had been injured. there, creekside, they saw a woman -- nancy mason -- lying unonscious by the water's edge. lynn, who has some emergency medical training, checked for vital signs. >> there was nothing, no sign of any pulse. >> reporter: lynn raced up the hill to the general store in the ghost town of st. elmo to call ai91ag1 n and give a better location. shortly, emergency responders arrived and began working on nancy. judy and lynn consoled dan. >> just to talk to him and -- and be compassionate. >> reporter: dan told the couple

what happened -- that he and his friend had been fishing downstream while nancy had wandered off to find a spot higher up. next thing he knew, there was a scream, and she was in the creek. he said he'd nearly drowned trying to save her. >> and he was asking questions, you know, "is she gonna be all right? how's she doing?" >> reporter: what'd you tell him? >> i said, "she's doing fine. she's gonna be fine." >> reporter: but those were words uttered by a good samaritan simply trying to say the right thing. the truth was, nancy was gone -- declared dead at the scene. the county coroner would later report she'd suffered a broken neck, the victim of a deadly fall. wes, how'd you find out your mom had died? >> i got a phone call from my brother ben. he said, "there was an accident. i need you to get down to your brother's house." >> reporter: wes raced to se his oldest brother matt in person. >> and he opens the door and shuts it right behind him. and his eyes are as big as saucers.

>> he said, "you know, you look like you've seen a ghost." and -- and i just started crying, and, you know, i said, "well, mom's dead." and then he just started crying. a it was probably the hardest moment in my life. >> reporter: in the days ahead, nancy's family -- her children, siblings, even her ex -- gathered to comfort each other. her dad, bill -- >> we were in the fog, we just couldn't make sense of it, you know? >> it was just like a blur. >> they started telling us, you know, what had happened and they said that they were out fishing and that she fell down the side into a river. she didn't have the greatest balance. she didn't have the greatest feet. i can see her falling. >> reporter: and not all that unusual a death.

chaffee county, where nancy died gets its fair share of fatal accidents, hikers, climbers, river accidents like nancy's. for the arriving officers, it shaped up as a tragic but routine incident. keith pinkston is the chaffee county undersheriff. he says deputies at the scene that day found the spot where it appeared nancy had put a foot down wrong and slipped. >> as you can kinda see, where that depression is, that's where the displaced rock was supposed to be. >> reporter: dan mason returned home from the fatal fishing trip distraught. nancy's family recalls he wasn't much help with the funeral plans. >> and he acted so strange. he kept running out the door. and all he could say was, "she wanted to be cremated." that was all he would say. he would not take part in any kind of planning. >> reporter: but how is a man who has lost his new wife supposed to behave? nancy's parents admit they never much cared for dan but believed he had the right to grieve, as they each did, in his own way. >> we included him with us. he sat right with us during the service.

>> reporter: he's grieving along with all of us? >> yes. >> yeah. >> reporter: but nancy's sons look back at their mother's funeral and remember a bizarre dan mason. >> you know, there was just him going, "oh, oh." like if you wanted, you know, to watch that, i'd probably turn o a soap opera. >> reporter: and after the service, the brothers say dan refused to answer questions about exactly how their mother had died. >> he said he was uncomfortable talking about it. he did not want to talk about it at all. >> reporter: but the sons say he did want to talk about her estate. not talk actually. he passed each of the boys an envelope with a note inside. none of them kept the letter, but recall it went something like this. >> "i'm grieving right now and all this stuff was our stuff. and i'd appreciate it if you guys would leave me alone." >> reporter: nothing sentimental about your mom and how much he loved her and -- >> no. it was, leave me alone and you'll get your stuff in due time. >> reporter: as it turns out, the boys would see very little

of their mother's estate. nancy had two life insurance policies -- dan collected a little more than $150,000, while her three sons split $54,000 among them. and, just a month before her death, nancy had signed this new will, leaving everything else -- the family home, jewelry, and whatever cash was left from her divorce settlement -- to dan. the will even went on to cite each son individually, saying they were specifically not provided for. it even mis-spells two of her children's names. it appears that nancy did sign the will, and the family didn't challenge it in court, but to the kids, it stunk to high heaven. >> with new people getting married, i'm sure she would have changed her will, but never taken out her kids, never specifically naming her kids out of the will, and our names mis-spelled on top of that. >> reporter: no one wanted to say it, but the thought -- nothing more -- was undeniably there. their mother who they say didn't

fish, who was afraid of rushing water. >> somebody in front of me said, "you know, what if it wasn't an accident?" >> reporter: not an accident? maybe things hadn't been so blissful for the second-time-around newlyweds perhaps three under a roof in a new marriage had been getting a little claustrophobic. as nancy's family tells it, not long before the fatal fishing trip, dan had gotten a sharp message from his new wife concerning their young friend living in the basement. >> and she said, "i'm just so sick of him being underfoot all the time." >> and she wanted him evicted from their lives, eh? >> she wanted him out. uh-huh. >> reporter: but still, nancy's death not accidental? her parents weren't hearing any talk like that. subject closed. >> it wasn't a popular subject among the family to talk about you don't want to believe it was not an accident because then what?

>> reporter: well, then you'd need to involve the sheriff's office down in chfee county. that's where undersheriff pinkston was having a rethink of his own as he looked over the file on that accident out at chalk creek. something seemed not quite right. coming up -- >> she said, you shoved her off that cliff, didn't you? what's nice about the iphone, is if you run into a friend and you want to share a photo

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nancy had been gone for four months, and her mother miriam refused to believe her daughter's death had been

anything but what the county coroner had ruled, an accident. she'd apparently fallen off a rocky ledge while fishing a fast-moving mountain creek, with her new husband dan. >> we didn't like him very well. but, we believed his story. >> reporter: is he staying in your life, or is he gone? >> oh, no. he wanted nothing to do with us. r:or r mllteiriam says she wanted from dan were a few family heirlooms, pieces that had belonged to her mother. but dan fired off a letter to his in-laws, reminding them that "my own life was almost lost on that day." and wondering if the family thought "tangible goods were more important than the demise of my wife." >> and he kept telling us how much he was grieving, how his life was upside down. and we were just driving him crazy because we wouldn't let him grieve. >> reporter: frustrated, they decided to drop by and retrieve the things unannounced. nancy's son wes went with his grandparents to help. there in the garage of his one-time home, he was surprised to see a flashy set of new wheels.

>> and so i looked in. i'm like, oh, that's a bmw-z3. >> reporter: that's the little two-seater? >> yeah. >> reporter: nice toy for a guy? >> yeah. they're pretty expensive. >> reporter: when they rang, dan's friend efren answered the door. the widower himself met the family inside. >> my grandmother was very stern and said, "you know, i'm here to get the antique table." and he was like, "you know, this is really hard on me, really -- you know, really hard on me." >> reporter: choking on his grief, all but paralyzed, was the way he came across that day. but to the family, it also emed that dan mason was moving on with his life. a good chunk of cash had apparently been splurged on redecorating. he and efren were making a bachelor pad. >> there was all brand-new furniture, beautiful furniture. nancy had never had new furniture. there were all kinds of things on the walls, like masks and swords. and there was a big bar set up. there was no sign of her anywhere. >> reporter: wes did end up hauling out the furniture they'd

come for. but as miriam surveyed the home that she says had been cooly scrubbed of any trace of her daughter's existence, the following words spilled out of her mouth -- >> she said, "you shoved her off of that cliff, didn't you?" "well, you know me better than that," he says. >> reporter: you confronted him directly? >> oh, definitely. >> reporter: how long had these suspicions been taking root? >> it just right boom, right then. that was when the light came on. when we stepped in that house that day, everything changed. >> reporter: but they didn't have much to grab onto, just their overwhelming sadness and a devastating hunch of foul play. three hours south, in the chaffee county sheriff's office, the same flickering thought. something's not right here, had been chewing away at the undersheriff. keith pinkston had been off duty the day of the accident. a week later, he'd decided to take another look at the paperwork on the case. there in the file, he studied a photo taken of nancy's fishing pole. funny, he thought, how it came

to rest so neatly on a g after her headlong fatal tumble. the picture made his nose twitch. >> your first red flag in this case was the fishing pole? >> yes. it was too convenient that it would land in that position on that log, out of everywhere else around it. i mean, didn't land in the water. >>rertpo er: had someone carefully set the pole there? its handle still wrapped in plastic from the store, to make it appear there'd been an accident and not something else? >> to me, that was not a sign that there was a crime here, but a sign that it needed to be looked at closer. reporter: pinkston headed out to the creek site to take a look for himself. when he got there, more red flags. the fishing story itself was sounding fishy. >> this is all whitewater in here. and you'd see, if you throw a line in there, all it's gonna do is wash away right away. >> reporter: and there was something else he picked up on, something he heard, the roar of the rushing river

water. >> when i got up here, you can see that the water's pretty loud. >> reporter: according to the case notes, dan said he'd heard his wife scream from where he'd been fishing downstream. >> i don't think it'd be very possible to hear somebody scream 80 yards away downstream. >> reporter: pinkston called in rob martellaro, a seasoned investigator with the district attorney's office who now works for the sheriff. he had a look and agreed that the story told creekside that day didn't add up. >> it just was different from how it was explained, very different for me. >> reporter: they grumbled privately about the lack of good investigating that should have been done right away but wasn't. the arriving deputy, who was later fired, hadn't even interviewed, or gotten the phone number of the good samaritans on the atv. the couple who got to the creek even before the emts. >> from the very beginning i didn't feel right about the situation. >> something was wrong that day. >> reporter: no one had asked

them at the time, but it turned out, lynn and judy cleveland had been bothered by a lot of things they took in that day. for starters, they thought it was strange that dan and efren had driven off together to look for help. neither had stayed behind with the critically injured woman. you'd think one of them would. and what's more, they say both men were wearing jackets. and yet they'd left nancy uncovered, with one leg still submerged in the roaring creek. >> couldn't imagine leaving a loved one by the side of the creek with their foot hanging. just didn't seem logical. >> reporter: and there was e something else that struck them, and investigators, as suspicious. they had encountered the men several miles up this bumpy back road. but when they got to the accident site, the clevelands realized they were just spitting distance from st. elmo, the ghost town that, on a holiday weekend, was abuzz with tourists. that's where lynn had gone to redirect the emergency responders. if somebody's in trouble, that's where you go to? >> right. >> right. >> reporter: so why had dan and

efren made a left turn away from the creek and driven several miles up a washboard 4-wheelers' path? when, if they'd gone the other way, the road up the hill could take them into st. elmo and to help in about 45 seconds? >> it's very close. >> maybe they don't have maps. maybe they don't know the area. here's the road and we'll go left instead of right. >> could be possible but i don't feel it is because they'd just been through st. elmo. >> and in fact, based on their story, they were by there that morning. so they had to know. >> reporter: and there was one more thing in they noticed that seemed to defy common sense. according to the men's story, dan and efren were fishing downstream when nancy fell. >> they're not looking to fish. they got the gear. >> according to ephran, he's fishing. >> reporter: but no one at the scene reported seeing fishing gear on the ground in this spot. the clevelands, however, told investigators that when they arrived they saw dan's suv was all neatly packed with fishing rods and tackle boxes in the back. >> so you have to believe what?

that they had this horrible thing with nancy. and then it's, "oh, we gotta pack up." >> "yeah, let's get our stuff packed. let's leave her." >> reporter: does that make sense, any of that? >> when you investigate, you try to be objective going in to it to see. but, it doesn't make sense. none of it does. >> reporter: growing suspicions. to put the story of a fatal fall to a real test, a team of sherriff's investigators later staged an accident recreation. dropping a mannequin nancy's size into the creek fre wrhe she allegedly slipped. and tracking its flow to the spot about 70 yards downstream where the men say they pulled her ashore. to pinkston, the results were telling. >> when we did our recreation with the mannequin, she would have never gotten past this spot right here. >> you would have found her body right there? >> she would have been right here. or she would have been sucked under the rocks right there. >> reporter: the scene didn't look right, and now chafee county investigators were contacting nancy's family for more background. into the case file went their stories about dan spending nancy's estate, what the

family thought was his weird behavior, and their belief that nancy never would have been fishing alone on that ledge in the first place. the questions were piling up. had it been an accident or something else? coming up, a voice from the past. >> i believe that he is dangerous. he would threaten to take my life. >> reporter: someone who new dan mason very well has a dramatic story to tell. applebee's 2 for $20! real food at the right price! this is the primo stuff. one appetizer and two premium entrees.

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investigators from the chaffee county sherriff's department thought they were building a strong case for murder against dan mason and his friend efren gallegos. along with the holes in the story the men told about what happened that day at the creek, the lawmen thought they'd uncovered a pretty compelling motive for wanting nancy mason gone, a classic one. >> dan mason may have benefited from what happened. >> reporter: this is money? >> this is money. i would say roughly $300,000 when it was all said and done. a good chunk of change. >> reporter: in january of 2005, police served a search warrant at the home where dan and nancy had lived.

on computers there, they found evidence of internet searches for information on dealing with grief, losing a spouse. but the hard drives revealed othetopics of apparent interest -- escort services and prostitution. and two weeks before nancy died, police say there were yahoo! searches being performed for "people-plus-poison" and "silent weapons." the search of the home also turned up what investigators thought could be a smoking gun -- a document cops called a "script." on a bedroom nightstand they found a one-page narrative written from efren's point of view. it outlined the events as they'd already been told the day nancy died. as had already been told. to investigators, it looked like a sure sign the men were making an effort to keep their stories straight. but in colorado, and just about everywhere else, investigations don't result in arrests until the district attorney gives a case the go-ahead. in chaffee county, the procutor in the d.a.'s office

making that call was not convinced the sheriff's men had the goods. >> we explained to him everything we had, and they told us we needed more, we didn't have enough. >> reporter: after all, there wasn't a shred of ysalic evidence to present to a jury, no forensics of any kind to indicate a murder had even taken place. and remember, nancy's body had been cremated. the autopsy done at the time concluded her injuries were consistent with an accidental death. >> we went back to the drawing board. we got more -- what we felt was more. >> reporter: so in the months, and as it would turn out the years, to come, the sheriff's investigators continued digging, bringing each new tidbit to the attention of the d.a. they kept tabs on the men who, by then, had left colorado for texas, dan mason's native state. and the detectives talked to anyone they could find who knew dan. a story told by this woman, janet kiddy, merited special attention. >> i fearefor my children. i feared for mylf.

>> reporter: janet had been married to dan mason for 12 years before he met nancy. she too remembers being drawn in by dan, the life of the party. and, just like nancy, she says when they met she had some money and dan didn't. >> he kind of lived large, even if he didn't have money. >> reporter: as you look back on your history with him, do you think that little bit of money y had had something to do with why he was interested in you? >> i do. i do think that was why he was with me. >> reporter: but over the years, she says, dan grew abusive, violent. >> he would hit me. he would punch me. he would threaten to take my life. >> reporter: in one violent episode from 1996, janet told police that dan had tried to choke her. he was arrested for assault and later pleaded guiy to al misdemeanor harassment. and in 2003, after they'd divorced, dan was put in jail for contempt of court, violating the terms of their child support and custody agreement.

it was around this time that dan moved in with nancy mason. what did you hear about nancy? >> that she was just a friend. that she had a really nice home. she had a lot of money. >> reporter: did you ever feel like, maybe, picking up the phone and talking to this nancy? >> i did. there were times i wanted to reach out and say, "you need -- you need to run and you need to run fast." i just thought she would think i was this -- >> reporter: the angry, embittered wife, huh? >> right. and she wasn't going to listen. now i regret that. >> reporter: today, janet kiddy has a permanent restraining order against her ex-husband dan. >> i believe that he is dangerous. >> reporter: as they kept digging, the sherriff's office gave nancy's family regular updates, signs of hope. but as months turned to years, the d.a.'s interest in taking the case to the next level went from lukewarm to ice cold. >> we heard it was dropped, couldn't do it.

>> reporter: couldn't do it why? >> because there wasn't enough physical evidence. there was no -- not enough evidence to really take it to a jury. >> reporter: it's a long time ago. you don't have a body. you don't have a lot of things that you'd like to talk about in opening arguments if you're a d.a. >> every day in america, there are cases like this, and they move forward. some are successful. some are not. >> to be honest with you, dennis, if i could convince myself at any point during this investigation this was an accident, it would've made my life a lot easier. i cannot convince myself it's an accident, and i feel my own duty to pursue it now as a homicide. >> reporter: and pursue it they did, attempting a creative end run around their d.a. they took their case file with its 95 bullet-pointed red flags to the county coroner, the very official who had deemed nancy's death an accident back when, and convinced him to give it a hard second look. the coroner agreed, and in may

of this year convened a public hearing on the evidence, a rare procedure in colorado called a coroner's inquest. what was your understanding of what the goal of a coroner's inquest was in your case? >> to change the death certificate from "accident" to "homicide" or "unknown." >> reporter: a panel of six jurors was seated. their job, to render a verdict on how nancy died. >> that meant the next step. this was huge. >> reporter: over four days, dozens of witnesses were called to testify. subpoenas were issued for dan and efren, but the men did not appear. and there was no lawyer present to offer a defense. the jurors were taken out to see the accident site at chalk creek. and, there, a demonstration was performed. could dan have heard nancy scream from where he claimed to be on the river that day? the test went something like the one we tried for ourselves out on the creek. our "dateline" producer stood at the spot where nancy allegedly fell while we listened below, 80 yards away.

>> let's see if we can hear that scream. go ahead. >> help! help! >> do you hear anything? >> did you guys scream? >> reporter: back at the courthouse, the coroner's jury went off to deliberate. the question, had nancy's death been an accident or a homicide? coming up -- >> we went away thinking we're going to get some justice for nancy. >> reporter: but justice might not mean the same thing to everyone.

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almost five years to the day that nancy mason died, a coroner's jury deliberated for just minutes before coming to a unanimous conclusion. >> six jurors in 15 minutes decided it was not an accident. >> there it was in black and white -- a new death certificate declaring nancy mason the victim of a homicide. >> so, we went away thinking we're going to get some justice for nancy. >> reporter: good news for you and the family? >> great news. >> arrest warrants for her husband dan and his friend efren gallegos were issued. and police were dispatched to the dallas, texas, apartment complex where the two men had been living. >> they went and picked efren up immediately but couldn't find dan. >> reporter: as the texas search for dan continued, nancy's family made the three-hour drive

to salida to see efren who'd been extradited back to colorado. there, nancy's mother confronted him at his court appearance. >> i said, "ephraim, nancy was good to you and you killed her." and he just kept his head down. he never looked up. >> reporter: but it was effren, and by extension dan, who would prevail in court that day. despite the jury's ruling that nancy's death had been a homicide, the d.a. came to court and told a judge there's no case here. >> i believe that there's not evidence to file criminal charges at this time. >> reporter: thom ledoux has been the elected d.a. in chaffee county since just january, but he holds the exact same position on the case that the prosecutor's office has maintained for years. it's your core belief that the case as it exists now is a loser. >> it's not about winningr losing. there's not enough evidence to prove that nancy mason died as a

result of a criminal act. >> reporter: for starters, the d.a. points out that while the jurors may have believed nancy was murdered, the panel was undecided on the question of what actually killed her, was it a broken neck? or maybe a drowning? perhaps a blow to the head? they couldn't say and there were no remains to re-examine. >> deciding how she died is an important aspect of proving who killed her and -- and how they killed her. >> reporter: and where the investigators see suspicious behavior, the d.a. sees plausible explanations. the men left nancy alone by the creek because, they say, neither one of them knew cpr. what could they have done for her? and driving the wrong way to go look for help? maybe that's because they were distraught and disoriented. >> this was their explanation. we didn't know exactly which way to go. >> reporter: don't yoalways say to jurors, "look, rely on your common sense." >> i would. >> reporter: if it doesn't sound good to you the law would probably agree? >> there are certainly some suspicious problems with the circumstances as we know them -- but i just don't know if she died of an accident or -- foul

play. >> reporter: as for the money motive? the d.a. says it could be argued that nancy was actually worth more to dan mason alive then dead. she was still owed another $75,000 cash from the divorce settlement, and they were apparently living off her nursing salary and $3,500 a month alimony. what's more, the d.a. says a lot of the persuasive evidence the coroner's jury heard wouldn't be admissible in a criminal trial -- things like stories about dan's troubled first marriage. but even if he could put dan's ex-wife on the stand, the d.a. says the fact that they were married for 12 years might play to the defense's hand. >> it's not as if there is simply a pattern of marrying individuals for their money, and then -- attempting to kill them. none of the other women that mr. mason has been married to have died. >> reporter: ledoux also points out that, as with any cold case, memories can become unreliable. the clevelands, for example, weren't even interviewed for months and now five years have gone by.

>> did you guys scream? >> reporter: as for our entirely unscientific test that showed it would be hard to hear a scream over the rushing water? well, the d.a. says members of his own officials came too a different conclusion. >> the individuals all relayed to me that it was loud but you could hear someone scream. >> reporter: have you been up to the scene? >> i have not been there. i have relied on the on-scene observations of the members of my staff. >> reporter: and finally, that so-called smoking gun document investigators found in dan and efren's house, outlining the event of that day? wasn't that a sign that the two men were ting to keep their stories straight? what about the script of what happened? >> when dan mason was questioned about the script, he freely admitted he had authored the script. and he indicated that it was part of a therapeutic suggestion. >> reporter: therapeutic? >> i'm not a therapist. >> reporter: jurors would look at that and say, "that stinks. why do these two guys have a script, a common story that they've worked out?" >> i'm not sugsting that there

isn't something strange about the script. it does not prove conclusively that dan mason and efren gallegos murdered nancy mason on may 30th, 2004. >> reporter: in other words, lots of smoke maybe but not enough legal fire. >> we have a legal and ethical responsibility only to carry forward cases that we believe there was a crime committed, and that we can prove who committed the crime. we don't just roll the dice to see, you know, if something will stick. >> reporter: so after spending ten days in police custody, efren gallegos was let go. the search for dan mason was called off. but that wasn't the last of the case or of dan mason. we went looking for him, to find out what he had to say about that day on chalk creek. coming up -- >> if she was your mother, would you not try everything? >> reporter: aho on

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after five years of investigating, five years of smoldering suspicions, the chaffee county d.a. added all the "what ifs" and decided not to charge dan mason and his friend for the murder of nancy mason. in the end, he says it wasn't even a close call. >to be honest with you, it really wasn't a very difficult decision to make. >> reporter: to say that nancy's family is furious with him would be understating the case. >> i think that he's incompetent. i think that he's a cold, unfeeling -- >> the district attorney is supposed to protect the people of chaffee county. if you want to do away with somebody, i think that's a good county to take 'em. >> reporter: what would you say to the d.a. if he was sitting

where i am? >> what if she was your mother? would you not try everything? >> reporter: the lawmen, who have doggedly pursued the case for years, also respectfully disagree with their d.a. >> obviously there's problems with the case, but there's still a lot of good parts to this case, a lot of good evidence in this case. >> and there are a lot of maybes in this case, and -- you know, you can explain away any 3 or 4 or 10 or 12. but the makes just keep lighting up. >> reporter: as for the man at the center of the sheriff's investigation, dan mason, where had he gone and what did he think about the uproar he'd left behind in chaffee county, colorado? he'd been a wanted man for just a few days in late spring, you'll remember, but police had lost track of him. "dateline" learned that just about the time the cold case was heating up again in colorado, dan mason moved from dallas to

san antonio, where this woman, gloria donovan, tells us she was introduced to him as "dan reed." she knew nothing about the investigation in colorado when they met, but says she didn't trust him from the get-go. >> it was everything, his whole behavior. it was all fake and phony and deliberate. >> reporter: dan is currently dating one of gloria's close friends, someone she describes as both wealthy and niave. >> he wanted to marry her right away, and have her go to las vegas. >> reporter: we, of course, wanted to talk to dan, to ask him about his wife's death, his actions on that day and in the months that followed. so we went to the home where he's apparently living with his new female companion. she answered the door and told us dan wasn't there. but it turned out he was home and minutes later on the phone he said we could come in for an interview in exchange for money. well, we don't pay for stories. he didn't talk to us on camera, but has communicated by e-mail.

sending "dateline" this six-page essay -- "my story: harassed from colorado to texas." in it, he says he's an "innocent american citizen" "who is being bullied" by certain members of th"colorado legal system." as proof of innocence, dan mason cites the d.a.'s refusal to prosecute and the coroner's original determination that nancy's death was accidental. and he says he has repeatedly answered questions about what happened that day and has never changed his account of events. dan mason's story also accuses the chaffee county sherriff's department of leaking false information about him to the media and he says investigators attempted to bribe efren gallegos to "make up a story" against him, something the sherriff's department denies. we tried to talk to efren gallegos, but he declined to comment on the case. >> it's possible that two people that committed a crime are never going to answer for it. >> reporter: but there is one more card to turn over.

the case isn't dead yet, but it's a legal long shot. in a highly unusual move, the sherriff and county coroner have asked a colorado judge to appoint a special promise cuter to try the two men for murder. just today, they withdrew that request but say they plan to file it again after refing their legal strategy. >> at least we can go to the family and tell them we did everything we could. we took it as far as we could and we gave it everything we had for nancy and for them. >> she was smiling. >> she was pretty happy there. >> reporter: for now, nancy's family is left with her scrapbooks to refresh their memories. >> oh, gosh. i miss her every day, holidays especially. >> she was a good person. she had a lot to offer. >> a good girl. >> there's not a day that goes by that, you know, something doesn't remind me of her. you never -- you never know what you have until it's gone.

>> reporter: gone with a scream and a headlong plunge into a furious rocky creek. or maybe it happened anher way? that's the mystery of it all that's just eating some people up. the whitewater of chalk creek may roar, but no one can make out its words. >> and theres more on this story on our website. the address is dateline.msnbc.com. that's all for this edition of "dateline monday." i'm ann curry. for all of us here at nbc news, good night.