>> she came over for a christmas party she told me she loved me and gave me a big hug. and it was just beautiful. i'll always have that to hold onto. >> reporter: the next night she's murdered. a teen. left dead by the railroad tracks. >> they told me they had found her body. >> it looked like pure evil met pure innocence. >> who hated her that much? >> reporter: the only threat they could find threatening to cut sarah's hair five years later, just across the river, another murder a young mother. >> i heard the gunshots. >> they say i have her nose. there are so many things that i don't get to share with her. >> reporter: for decades both

cases were colder than a grave. >> you're always trying to make the pieces fit together nothing fit but one former cop couldn't let go. >> this is crazy this isn't solved. >> reporter: and one woman couldn't forget. >> the warning signs were there. >> reporter: two victims with one link in life. >> it was an obsession. it was control it's the same mo. >> reporter: could a break in one murder help solve another? >> things started fitting. we're getting to the end. >> reporter: i'm lester holt and this is "dateline." here's josh mankiewicz with "fury." >> reporter: the kansas river separates kansas from missouri. but it also divides more than that. there's a family on each side. and for a long time, each has been separated from the truth. and from each other. each thought they were alone in

wanting answers to the same questions. >> outraged that i've gone years without ever knowing -- connecting them and getting closer to those answers took more than a quarter century. >> unless you go through this, you have no idea what it's like. this is how it happened. and how it began. >> reporter: it was december 29, 1989, in kansas city, kansas. a train conductor found the body. right alongside the railroad tracks, under the highway. >> this just seemed like it was somethin' right out of a crime scene book. >> reporter: at only 19, police cadet jeff cheek had already been to a few homicide scenes. but now he was looking at the body of a girl about his own age. >> the body was behind the pillar. >> so hidden from the road? >> obvious attempt to hide her from the road. >> reporter: the victim had been stabbed to death.

her name was sarah de leon. she was eighteen. >> just horrible scene. it was -- it -- it just was horrible. it was just -- reeked of evil. >> reporter: jeff realized they'd gone to the same high school. >> i'd never talked to her. but i said, "i do think i recognize her." "i think i remember seein' her in the halls i just remember it hittin' me like a ton of bricks that this is -- this is a young girl. >> reporter: this is sarah de leon's mom, gail. >> she was -- just -- just -- a good, quiet, fun-loving girl. she was health-oriented. she didn't even drink caffeinated beverages. >> when you worried about her and every mom does, what'd you worry about? >> i was concerned about -- her going away, just growing up, going away to school. dating, meeting someone. >> reporter: sarah and her younger brother, matt -- now 44 -- were only a year apart. >> you guys get along? >> we got along really good. i played tennis at the high school and she would come to my practices and sit on the hill and watch me practice. >> reporter: and matt says he respected her for being a leader and not a follower.

>> people wanted to be like her, you know? she was, like, a trendsetter. she didn't care what other people thought. >> reporter: that december, sarah had just finished her first semester at community college. she was intrigued by other cultures and hoped to one day work in the travel industry. >> everything was coming together for her so well. it was just coming together so well. >> reporter: she had a serious boyfriend, even introduced him to the family. >> she and her boyfriend, came over for a christmas party. it -- it was wonderful. and she didn't want to leave. she stayed and she told me she loved me and gave me a big hug. and it was -- it was just beautiful. >> reporter: that wednesday night was the last time gail saw her daughter. friday afternoon, two police officers were at her front door. >> they came in and asked me if i -- was sarah's mother. "yeah." and -- they told me that they had found her body by the railroad tracks. and - - that she had been murdered. >> reporter: jeff -- the young

cadet -- didn't just know the victim, he also knew that spot down by the tracks. >> this was an isolated area. i told the detective "it used to be a party spot for kids our age and older." "and the reason why there were parties there is because they knew it was isolated from the police." >> so high school kids went out there -- >> right. >> -- to drink beer and smoke -- not to be caught. >> right, so immediately i was kinda thinkin' in my mind, "who else knows about this place?" >> reporter: gail could not fathom what her sarah might have been doing there. >> she didn't put herself in risky situations. >> you couldn't have imagined her doing drugs. >> no. >> or drinking. >> no. >> reporter: police tried to retrace sarah's steps. her boyfriend told cops they'd gone out for dinner that evening, and when sarah left his place in her car late that night, he said, he never saw or spoke to her again. and the next time anyone saw sarah de leon, she was dead. killed somewhere else and dumped by the railroad tracks.

>> somebody just pulled up, dragged her out of the car? >> right. that's what it appeared to be. it looked like the car stopped out here, and you could see the drag marks all the way around. >> reporter: there was nothing to suggest that sarah had been at those parties down by the tracks. but when word of the crime spread, jamie locke, 20 at the time and familiar with the local party spots, says it was terrifying. >> the eerie feeling was the fact that, you know, where she was found. >> the place where you guys went to party? >> yes. >> reporter: so, was it someone sarah knew or something random? jamie remembers that some thought that it might have been a "bump-and-rob." >> someone was bumping into cars, you know, little fender bender and then robbing them. >> after you pull over? >> yeah. after you pull over. >> we did have a number of those at the time. you get out of the car. and they go, "thank you. i'm takin' your car." they leave you there. >> reporter: so, was this a "bump and rob" turned

bump-and-murder? sarah's car was found abandoned at this overpass, miles from where her body was dumped. >> the police did take sarah's car, put it up on a rack, and look at the springs and the shocks to see if it looked like it had been bumped. >> that does kinda fit with some things that were going on at the time. >> yeah. that would fit. i mean, it's out there. the police are considering it. but that didn't fit, for me. >> reporter: it didn't fit she said, because detectives told gail how many times sarah had been stabbed. too many to count. >> the passion in this murder, i -- who hated her that much? >> 'cause that's fury. >> it was real fury and hate with a passion. that was personal. >> reporter: it was that mother's intuition that would fuel a search for the truth. it would take many years and it would lead to the other side of

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>> in the days after 18-year-old sarah de leon was murdered, kansas city kansas police worked around the clock to find her killer.

jeff cheek, then a cadet, says the pieces weren't falling into place very easily. >> they didn't find a weapon. >> reporter: no fingerprints? >> couldn't get fingerprints from anything. >> reporter: and no witnesses. >> and no witnesses. >> reporter: what do you do? >> i know they were looking at robberies. whether it was -- kind of a process of elimination. friends -- threats -- you start developing motives. you start talking to people in the inner circles. that' s -- that's really how you work that kind of a homicide. >> police briefly considered the possibility that sarah's death had been the result of a "bump-and-rob" car-jacking. to jeff cheek, that didn't make much sense. >> in this case, they left a perfectly good car behind. bump-and-robs don't do that. they don't kidnap you, take you somewhere, kill you -- and then leave a perfectly good car behind. >> so they started working through sarah's inner circle. first up, her boyfriend matt uland. >> reporter: did you know her boyfriend? >> i met her boyfriend and, and i -- didn't spend a lot of time with him. he seemed like a -- good guy. >> her brother remembers talking

to matt in the hours before sarah's body was found. >> sarah's boyfriend -- called the house. and i said, "hello." first thing he said was, "do you know where sarah is?" so, you know, i was, kind of, like, "no." >> they questioned him. of course -- that's one of the first things you do. who's the last person to see her alive? >> and his answer was, "i don't know anything about this." >> he said he'd spent the -- the night with her. they had went to dinner. >> the boyfriend told police that after sarah left his house around midnight, he was home all night. and his mother backed up that alibi. >> matt, her boyfriend. >> uh-huh. >> reporter: do you suspect him? >> i didn't. and i didn't know him that well. but he'd just been at the house. he seemed like a decent kid. his mother said that he was home at the time. i -- i believe that. i didn't see him as -- as a suspect. >> reporter: but when that happens, you got to start looking at everybody in her life. >> you do -- >> reporter: thinking maybe i've -- maybe i've missed something. >> you do.

everybody -- i'd go to the store and everybody there was a suspect -- the back of my mind -- it was like a computer going 24/7, trying to fit the pieces together with what little i had. >> sarah's friends confirmed what her family had said: sarah was popular. a leader. a good kid and a good student. police did hear one weird story from a few kids in town -- about a girl who asked a male friend to help her do something mean to sarah. >> reporter: somebody had encouraged this guy to put something in sarah's drink? >> yes. >> reporter: to what? >> at a party. >> reporter: knock her out? >> yes. >> reporter: so that? >> she could cut off her pretty little hair. that's what i got. >> this girl who police heard wanted to drug sarah and cut off her long hair? she was sarah's boyfriend's ex -- >> there was some animosity there. >> reporter: jealous. >> jealous. jealousy. >> the police asked me if i'd heard of this gal. i said no. but apparently she had threatened to cut sarah's hair. >> was sarah's death some kind

of sophomoric prank that went too far? it seemed crazy, but police went ahead and interviewed the girl. it led exactly nowhere. >> meanwhile sarah's mom did everything she could to help the investigation. even going on a tv show called "missing reward" to plead for information. >> somebody has to know what happened. and i'm just asking for them to let the authorities know. it's hard to come to terms with the death of your child when you don't really know what happened. >> before long, the investigation seemed as cold and empty as the place where sarah's body was found. >> i was told there was just nothing. >> reporter: and you get the feeling the police think we're not going to solve this. >> they had no -- no clues is what i was told. >> reporter: could you live with that? >> i was living with that, yes. that we would never know. >> months, and then years. jeff was still on the police

force. although not assigned to the case. >> i talked to the detective i'd say, "hey, what happened with the de leon case? did you get her -- did you get that solved? he said, "no," you know, "we haven't got it yet." >> reporter: disappointing. >> oh, yes. very disappointing. >> gail faced life every day without sarah, and without answers. and that lasted five years. that's when another family in the same area found itself in the same darkness. another young woman was dead. another case with no answers. coming up, a mother murdered. >> she was walking towards the kitchen. that's when i heard the gunshot. >> and a husband with a secret when dateline continues.

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>> reporter: sarah de leon's murder had been cold five years and now once again, it was a frigid night. super bowl sunday, 1994. just about everyone had the day off. except, perhaps, for the cops on the night shift. patrolling independence missouri, across the river from kansas city. they spotted a car around midnight. lights on, engine running, abandoned in a church parking lot. a quick check of the plates told them the car's owner, 26-year-old diana ault lived in house a mile away. >> they say i have her nose and her hands. that's the best compliment really. >> reporter: katie ault is diana's daughter. she's now almost the same age as her mother was back then and like her mom in more than just looks. >> she used to work several jobs

and do direct sales. she was always so busy and a lot of people tell me i'm the same way. >> just lookin' back at pictures it does bring back -- back memories, >> reporter: this is diana's son, josh. >> my dad worked nights, and sometimes she'd let me stay up watchin' tv and, waitin' for dad to come home. >> reporter: dad was tim ault. he worked at a postal service bulk mail center and was pulling a night shift that super bowl sunday. josh was just 4-years-old, his little sister katie in diapers. diana and the kids watched part of the game with family, the dallas cowboys versus the buffalo bills, and then headed home. >> reporter: tell me about that night. >> i remember coming home and it being dark. i remember going into the house, heard my mom going back out to the car for something, whether it was groceries or a bag -- diaper bag. >> reporter: kid stuff. >> yeah. >> reporter: some of josh's

memories are fuzzy, but not what happened in that very next moment. >> as she was walking through the livin' room to -- towards the kitchen. that's when i hea -- heard the gunshots. >> reporter: gunshots. and then, four-year-old josh saw the shadow of his mom collapsing onto the kitchen floor. police later theorized the shooter had been waiting inside the house when the family got home. >> very quickly after, i was picked up by somebody. i can't remember the details as far as what the person was wearing or the body structure or -- >> reporter: whether it was a man or woman? >> yeah, i -- i -- i can't remember. >> reporter: he says the person who scooped him up wore a mask. and then gently carried him down the hallway and into his parents' closet. >> it wasn't like they just picked me up, threw me over their shoulder, and threw me in the closet. i mean, th -- >> reporter: they -- they were holding you like -- like -- like you'd hold a baby. >> right. >> reporter: how long were you in that closet? >> i don't know. but i do remember coming -- finally working up the courage

to come out. and i can remember seeing my sister crawling on the floor in the area of my mom. and i've been told that i tried to make my sister a bottle or comfort her. >> reporter: the killer had left in diana's car, so when police found it in the church parking lot and then traced it back to the owner's home, they found a four-year-old giving a bottle to his baby sister. their dead mother lying next to them. >> grandma told me that i had a really brave older brother, and that even after everything that happened, he was still trying to take care of me. >> reporter: late that night, diana's father got a knock on the door. >> so as i opened the door, tim's father was standing there. and the first thing he said when i opened the door was "diana's been hurt. and she's dead." >> reporter: he immediately called his other daughter, sharon.

>> i just remember sliding down my stove, just with this gut-wrenching scream cry. it was -- i'll never forget it. >> reporter: sister sharon and her dad, and diana's husband tim, rushed to the police station, where police asked tim the obvious question. who would want to kill your wife? tim said he had no idea. one more thing, the murder weapon was one of tim's guns, taken from the house and left in diana's stolen car. tim said none of it made sense. >> tim was sayin' that they had a good marriage, everything was good, they got the two kids, and things couldn't be better. >> he couldn't think of anyone who would want to hurt her. he didn't understand why this happened. >> reporter: detectives told the family the working theory was a robbery gone bad. but diana's sister responded by telling the police something they did not know. she said tim and diana did not have a great marriage. in fact, she said, tim had been cheating on diana. and just a month earlier on christmas eve, he'd left her for

the other woman. >> she was very upset about it. so -- and i -- i just remember thinking, "on christmas eve? really?" >> reporter: that other woman was a co-worker of tim's from the postal service. diana's sister sharon also told the cops that after a very unhappy holiday season, tim had ended the affair and moved back home. >> this detective sat there like he was just shocked. he hadn't ever heard this. and i ask. i said, "is tim still here?" and he said, "yeah." so i said, "go ask him if what i'm saying's true." and he confirmed it. >> reporter: so police spoke with both tim and his mistress. questioned both of them. tim's alibi checked out. he was at work. and the other woman's alibi was good, too. she said she was more than 100 hundred miles away. the case remained unsolved. diana ault's children were raised by their grandparents and

their father, tim. >> i remember asking josh, you know, why we didn't have a mom. and, you know, all your friends, you go to their house, and -- and they have moms. but he was the one who kind of first said, "she's not here because someone took her." >> reporter: so now fast forward twenty years. diana ault's children are grown. son josh newly-married. across the river in kansas city kansas, frustration is a feeling that the other family, sarah de leon's family, has grown used to. >> i think they went on to other crimes. and that was just kind of on the back burner. >> reporter: and jeff cheek is no longer a cop. but he never forgot about the crime scene down by the railroad tracks. and then, one day the local news covered a vigil. turns out, the tv was on in exactly the right home. >> my eyes popped open and i sat up.

i know that case. coming up. two murdered women with one link. some wondered is that what linked them in death. the same mo. i want this person. i don't want you to have this person. ething... set it free. see you around, giulia ♪

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>> reporter: it was january, 2014. jeff cheek was home, the tv on. >> i was just getting ready to fall asleep. >> reporter: sarah de leon's family was holding a candlelight memorial. >> i waved goodbye to her. >> reporter: the local news turned out to cover it. >> and i just heard this 1989 case, a young female.

>> reporter: for just a second, jeff thought maybe someone had solved sarah de leon's murder. >> but i quickly was disappointed in seeing, oh -- [ sighs ] it's just a vigil. >> reporter: by then, jeff had left the kansas city, kansas police force, and he couldn't believe the case still hadn't been solved. he decided to reach out to sarah's family and work for them as an investigator. >> i could tell as soon as i met jeff, he was just a good-hearted guy that just wanted to do the right thing. >> jeff, our little angel, gets online and starts thinking there's got to be some new technology. >> i did a ton of research. i talked to people all around the united states. i talked to the best forensic minds. i talked to cold case bloggers. >> reporter: he also began looking for witnesses. and one of the people he spoke tfwith was jamie locke, the woman who'd grown up in the same area as sarah de leon, and used to go to those parties down by the railroad tracks. >> the place where the police didn't come that often? >> right. >> reporter: jamie told jeff she didn't know sarah personally, but she did know sarah's boyfriend, matt. and, she knew the girl who dated matt before sarah. her name was carol coon. >> was she your best friend? >> she was one of them, yes.

>> reporter: that friend carol is the same young woman who was briefly questioned by police back when sarah was killed. that's her on the left in the overalls. >> what was she like? >> she was smart, focused, driven. >> reporter: and says jamie, carol was determined, very determined. here's one example. carol was a fan of those radio station contests where you could win a car by physically keeping your hands on it longer than anyone else. carol won a car that way, not once, but two years in a row. >> that is someone with some significant willpower. >> yes. but i knew. i knew she would win. i knew. >> reporter: jamie says that through their long friendship, she noticed carol was always determined to get what she wanted. and when she didn't, jamie says carol sometimes played dirty with her romantic rivals. >> she was seeing a guy, and he

had a girlfriend. she would stop by the girl's house, and let her air of the tires. >> let the air out of the other girl's tires? >> yes. >> reporter: and jamie says her friend carol saw sarah as a rival. she says she heard carol talking about wanting to drug sarah and cut off her long hair. >> was she serious about this? >> i didn't know if she was serious. >> this is a little beyond letting the air out of somebody's tires. >> yes, right. >> this is past a prank? >> right. >> it's a little scary? >> very scary. >> reporter: which is why, for a split second after jamie learned of sarah's murder, she wondered if her friend carol had somehow been involved. >> clearly, she was jealous of sarah and up -- and upset. >> reporter: but jamie had no proof that her friend did anything at all to sarah. in fact, after the murder, carol seemed even more afraid to go out than jamie. >> carol said that, "we should be careful because, you know, this could happen to us.

let's be really careful." >> reporter: and jamie tagged along when carol was questioned by police. >> she comes out. she's smiling, giddy. >> not a care in the world? >> right, and i think then i was -- like, well, obviously, they don't think that she had anything to do with it. >> so you shouldn't either? >> i shouldn't either. >> reporter: but jamie also says she noticed something odd the morning after sarah's murder, something she told jeff about. >> she had a scratch on her neck. >> did you ask her what that was from? yes. she said her cat scratched her. another thing. carol's car was -- carol was always sloppy, messy. and her car was spotless. it was clean. i think in my gut, you know, i was like, "this -- something's not right." the warning signs were there and i just chose to ignore them. >> reporter: she ignored any doubts she might have had for years. but jamie told jeff all that changed the moment she heard about another murder victim, diana ault. >> did you know diana? >> no, no.

i knew of her. you know, i knew that this was tim's wife. >> reporter: and how did jamie know that? well, it turns out that tim ault's mistress. that co-worker from the post office was also jamie's friend, carol coon. >> she was saying this is coincidence just a coincidence. how could this be a coincidence? no. >> reporter: jamie says she knew all about the affair between carol and diana's husband, tim, and how certain carol was that tim ault would permanently leave diana for her. >> this was the man she was going to marry. after all these guys, you know, one -- one's sticking around. one's staying. >> but then he didn't. >> he didn't. >> reporter: jamie says that when tim ault ended the affair and moved back in with his wife and kids, her friend carol was devastated and furious. according to jamie, carol asked for help in what jamie thought was an effort to get tim and his

wife to break up. >> she asked me to call diana. she wanted me to call her and tell her that he's with me. >> that you're having an affair with tim -- >> that i'm having an affair with him. >> even though you weren't? >> even though i wasn't. >> you didn't say to her, "leave me out of this"? >> i d -- i did. but she was persuasive. i mean i didn't want to. >> reporter: jamie says she ultimately gave in and made the call. >> and you said, "i'm having an affair with your husband. he was with me last night." >> yes. she hangs up the phone, and i hang up, and that was it. >> reporter: that was on a saturday. the next day... was super bowl sunday. and the day after that, monday morning, jamie says, carol came to find her in person. >> and she said, "sh -- she's dead." i said, "who's dead?" she said, "tim's wife is dead. she was murdered." and i said -- [ sighs ]

"did you murder her? did you kill her?" >> that's the first thing you thought? >> i said, "did you kill her?" and she said, "no." just like that. >> reporter: but as jamie tells it, that wasn't the end of their conversation. >> she said, "whatever you do, do not tell them anything about making that phone call to diana. there's no way they can find that out. don't tell them." >> so your friend's telling you about the murder? >> uh-huh. >> saying she didn't do it, and telling you essentially to start concealing evidence. >> right. >> reporter: and now a private investigator was taking notes on what sounded to him like a deadly pattern, two dead women with one jealous rival in common. >> it's the same mo. it's -- i want this person. and i don't want you to have this person. >> reporter: but here is perhaps the craziest thing about jamie locke's story. this wasn't the first time she was telling it.

it was way back in 1994, right after diana ault's murder, when jamie says she told police she suspected her friend carol might be responsible for both crimes. >> these are two murders, five years apart, two different jurisdictions, two women killed in different ways. and the police don't put it together. you do. >> yeah. >> reporter: and what happened next? for two families, that was the real mystery. coming up, a loving parent as well as a murder suspect. when date line continues. in t,

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>> reporter: jamie locke suspected the connective tissue in two unsolved murders was her good friend, carol coon. and years before retired-cop-turned-private investigator jeff cheek ever contacted her, jamie had shared her suspicions with police. >> you told police that you thought your friend carol was responsible for diana ault's death. >> yes. >> and sarah de leon's death? >> yes. i mean, i was still having a hard time believing that, how could she, you know, live this double life.

but i needed the police to -- to look into this. >> reporter: jamie came forward not long after diana ault was murdered back in 1994. she says she told police about what she saw as her friend carol's history of seeking revenge on romantic rivals. like how she says carol let the air out of one girl's tires and later threatened to cut sarah de leon's long hair and how carol asked jamie to make a harassing call to diana ault right before her murder. >> i mean, i know it was escalating. but i never thought that she would be capable of -- of hurting someone. >> reporter: jamie says that back in 1994, she believed police took her seriously, but she worried that the phone call she made to diana made it look as if maybe jamie knew more than she was letting on. >> let me ask you.

any part of what you're telling us not true? >> no. >> any part of it shaded to make you look less guilty -- >> no, not at all. >> you participate in or witness any act of violence involving carol and another person? >> no. >> reporter: jamie says she told police investigating both cases back then that she would do anything to help. >> police wired up your phone and had you call carol. >> yeah. they wanted me to, you know, to basically to get her confess to me. >> how'd that go? >> she didn't say much. >> reporter: back then, tim ault also wore a wire while speaking with carol. apparently, she didn't say much that time, either. >> she was aware that police suspected her? >> yes. >> and so maybe she was being careful what she said. >> i don't know. >> reporter: back when jamie first came forward, sarah de leon's mom, gail, heard there was a lead in her daughter's case. >> i think i got a call that someone came forth and gave them a name. and that name was also connected with sarah. >> this was a woman who had

dated sarah's boyfriend, and had an affair with diana ault's husband. >> right. and the one who threatened to cut sarah's hair. >> reporter: gail says that to her, something just clicked, and she wondered if police had finally found the person who'd stabbed her daughter to death. >> it fit the rage. it fit the amount of rage in that attack. >> but? >> but the police looked at that a bit, and nothing came of that. >> reporter: what did police do back in 1994? we don't know. police in two departments aren't saying. cop-turned-p.i. jeff cheek says the lead detective on the sarah de leon murder, did try to assemble a circumstantial case. >> our detective submitted it to d.a.'s office, and the d.a.'s office refused to file. that's the information that i got. >> reporter: what we do know is that two families lived through two decades of frustration and unanswered questions. and carol coon?

she was never charged with any crime. she graduated from college, got married, became carolyn heckert, and had two daughters. >> she doted over children. she seemed like a good mother. >> reporter: cindy cygan met carol in 2013. >> the things she would say that she would do with them. and the -- lessons that she tried to instill in them, don't give up. you know, you have to keep trying. >> reporter: carol moved to the suburbs and became a successful real estate agent. and from the outside, at least, her life seemed to be working out just fine. >> she was very ambitious. she was a go-getter. she didn't have a fear in trying new things. she didn't worry about what people thought of her. >> reporter: angie loveless says carol tried hard to find her the perfect home, while being incredibly patient and never losing her temper. >> i'd put an offer on another house. and then i decided i didn't want

it. so i texted her, "please don't kill me. i don't want that house anymore." she's, like, "oh, no. it's okay." so -- >> she didn't get angry? >> no, i've never seen her angry. >> reporter: so which was the real carol? a homewrecker? a homemaker? a homicide suspect? jeff cheek was coming after all of them. >> i wasn't prepared to let it go. i got a lot of heat -- [ laughs ] from my old friends. but, it's the right thing to do. coming up. decades later. still more frustration. >> at times i felt like we were the enemy. >> and then, is this really happening?

>> reporter: two murders, decades cold. one common ingredient in both victims' lives, a woman who'd been a romantic rival of both. jeff cheek developed a theory of what had happened. >> was this, you know, "i have to have this guy"? or was this, "i don't like coming in second. i can't lose"? >> it was an obsession. it was control. it was, "things are going to be my way. nobody's going to have you if i can't have you." >> reporter: jeff spent months reinvestigating the sarah de leon case, interviewing witnesses and tracking down old leads. he knew there was evidence like sarah's clothing, which now, many years later, could be submitted for testing. jeff researched cutting-edge dna science. >> you're doing this for no money. >> here's the thing.

i feel so bad because my old profession, something that i used to represent has let these families down. how dare i take any money from them. >> reporter: jeff took his findings to his old colleagues in the kansas city, kansas pd. in 2014, they agreed to re-open sarah de leon's case. for the first time in decades, sarah's mother, gail, had some hope. but when she met with police detectives, she says it didn't go as expected. >> i went in there so naively, talking to the police and the assistant d.a.'s like, "we're going to be on the same team. we've got the same goal. this is great." it didn't work like that. >> reporter: she says 9 months went by and dna tests still weren't being done. and two years passed before jamie locke, the woman who first connected both cases, says she was re-interviewed. >> we come to them with names, "this is who you need to talk to. they've got some good

information connected with this case." and we are just discounted. at times, i felt like we were the enemy. >> reporter: across the river in independence, missouri, a detective told diana ault's family they had re-opened that case, too. but then, nothing. >> if you're going to make it known that, "hey, i'm going to look into this," then that needs to be what you do. >> reporter: so now jeff spoke with dozens of people in diana's circle, many of whom had never been interviewed by police. but who he says told him carol coon was waging a campaign of intimidation and harassment against diana ault in the weeks and days leading up to diana's murder. her sister says diana told of carol repeatedly making taunting calls to the ault house. >> carol had called her and said she had checked on social security to find out death benefits that she and tim would receive, that she intended to raise her children.

i told her -- i said, "you know, something bad is going to happen to you or the kids." and she says carol made it known she'd been "inside" the ault house. diana said carol left proof. and she got home. there was writing on the li -- mirror of the bathroom in lipstick. you know, basically saying that this person had been there in her home, and had sex with her husband. >> she left notes. she left a tip kind of thing, like, "hey, you know, thanks for your husband." it was very eerie. >> reporter: her sister says diana was fearful, telling her family she'd changed the locks and her telephone number. >> she brought the kids back to my place, went to the police station, made a report. >> reporter: jeff found a note in a police file showing diana had reported the harassment to law enforcement. >> she reached out to a lot of people. she -- she wanted protection.

she wanted help. two weeks later, diana's dead. >> reporter: back in kansas city, kansas, sarah's family says they realized they had been far too passive for far too long. with jeff's help, they kept the pressure on. and eventually sarah's clothes were sent off to a lab for testing. >> you lost some friends that over this. >> oh, i did. there's no doubt. i did. they shouldn't have put me in a position that we had to fight. >> reporter: gail says she learned a thing or two about dealing with police and how to make sure they didn't forget about her daughter's case. >> don't be afraid to call them. make yourself known. >> be a nuisance. >> yeah, i was. [ laugh ] i still am. [ laughs ] >> reporter: and maybe that determination paid off. this past may, brought a tantalizing announcement. >> everybody ready? >> reporter: kansas city, kansas police announced they had identified a female suspect in

the sarah de leon case. they didn't give a name, but if you knew the story, you knew who they were talking about. >> missouri authorities believe this case is linked to the january 30th, 1994 homicide in independence, missouri. we have also determined that the suspect has been involved in multiple instances involving the harassment and intimidation of romantic rivals. >> reporter: the big moment came five months later in october, 2016, nearly 27 years after sarah's body was found in that desolate spot down by the tracks. >> charges filed against carolyn heckert. >> reporter: carolyn heckert was arrested and charged with the 1st-degree murder of sarah de leon. >> it was excitement, but it was shock. it was, "is this really happening?" >> reporter: sarah's brother, matt. >> i never thought i'd see the day. i never thought i would hear those words. >> reporter: what finally connected the dots for police?

they're not saying, yet. >> feel like you're at the end of a long road? >> until there's a trial, i'm not going to feel we're at the end of the road. >> reporter: carolyn heckert has pleaded not guilty to sarah de leon's murder. her defense attorney says he's seen the state's evidence and there's nothing physically linking her to the crime. and, he says, that foreign dna was found on the victim, but none was from his client. the prosecution isn't saying exactly what the dna results are only that dna will not exonerate ms. heckert. she's been extradited from missouri to kansas where she awaits trial. and jeff cheek gave prosecutors in diana ault's case a 70-page report he'd prepared. prosecutors examined it, but say they don't have enough to go forward. there are no charges against carolyn heckert in diana's case. and so, the ault family's fight for justice continues. >> we're in this together with the ault family and, you know, we're only halfway there right now. >> reporter: it's been a slow, meandering journey.

two families, on two sides of a river, separated from the truth for so many years. it's not over yet, but hope runs deep. that's all for this edition of dateline. see you tomorrow at 8:00 p.m. central dateline saturday night mystery, and, again, sunday at 7, six central. i'm lester holt. from all of us at nbc news, good night.

a warning about falling ice in center city. thieves target a local barbecue restaurant. the $30,000 crime that's threatening business. an nbc 10 exclusive. volunteers rescue a goose attacked in a local neighborhood. the search for the shooter who opened fire on the domesticated birds. smoked out. thieves target a barbecue smoker. >> it's worth $30,000. for the owners of fat jack's barbecue, it's priceless.