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she really shined as a mother. everything was about her kids. i can't even describe to you the feeling that came over me at that point. we were completely dumbfounded.

ever -- could ever do that. >> they met on the job. >> they were an office place love story. >> and that's where she met her end. >> the blood is right by her office door. >> a lot of blood and a lot of trauma. >> a mom of two murdered at work. >> she wakes me up, kisses me goodbye, and that's the last i saw of her. >> she'd gone in extra early that morning. just a handful of co-workers in that building. was one of them her killer? >> i've referred to this case as almost being like the game of clue. you have a very small group. we know it happens within the building. >> what secrets were inside that warehouse? >> there were pry marks on her office door. somebody moved her body. >> room to room, co-worker to co-worker, suspect to suspect. >> the perpetrator could still be there. >> where was her husband? >> you're a suspect and so are your friends. >> the detective asked me if there was any trouble in their marriage. >> whodunit?

desperate things. >> i'm lester holt, and this is "dateline." here's josh mankiewicz. >> reporter: they call it the early shift for a reason. it was still an hour before dawn in this small town in northern kentucky, and in a few hours a local warehouse would become an anthill of activity, as the first employees of the morning arrived. but who among them could predict that before the first coffee break that day one of their co-workers would be dead? >> i just walked in our office, and i think somebody has killed somebody upstairs in our office. >> okay. what makes you think somebody killed somebody? >> she's laying there on the ground, and there's blood all over. >> reporter: impossible to believe in this tight-knit

everyone, but there was michelle mockbee, 42, wife and mother, face down in a pool of blood. the sheriff's department would launch an investigation. you know that your suspect is one of those people in the building? >> that's right. >> reporter: probing for clues in the victim's own private life. >> the detective asked me if there was any trouble in their marriage. >> reporter: eliminating suspect after suspect until there was one. >> he was never on our radar until we saw his truck leaving. >> reporter: if you give michelle mockbee's siblings a chance to tell you about their sister, they can't say enough good things about her. >> i don't think there's enough tape to talk about michelle. she was our big sister, our role model. >> michelle was just the most amazing sister that you could ever ask for, very loving, caring, giving person. she would do anything for

>> she always had a big smile, and her laughter was contagious. >> reporter: michelle carried her positive spirit into the workplace. she was head of payroll at that warehouse owned by thermo fisher scientific, a worldwide supplier of laboratory equipment. it was also where she met her husband, dan mockbee. what was she like? what drew you to her? >> michelle was funny, attractive, intelligent. >> reporter: i've heard you say that pictures kind of don't do her justice. >> no. no. no. michelle was much more -- there was a vibrantness to her. i mean, she was beautiful. >> reporter: it was at a thermo fisher christmas party back in 1999 that dan first summoned the courage to ask her out. you'd been thinkininabout mimielle? >> oh, absolutely. i'd been working on her for a year, so. >> reporter: he asked michelle to stick with him that night.

sideways. >> it was the worst date in the world. >> reporter: what went wrong? >> i don't know. i was totally off my game. i couldn't speak. and i thought, "oh gosh, i've done screwed this up totally. this was terrible." but then i asked her out again and she said yes and -- >> reporter: well, maybe you did something right. >> i must have. i don't know. or she felt sorry for me, gave me another chance. i don't know. >> reporter: michelle's family soon met him. >> next thing you know, they're seeing each other and getting pretty serious. >> reporter: after she came along? >> oh, my gosh, world totally changed. i mean, sun got brighter. it was just a totally different world. >> reporter: michelle and dan married in 2001 and continued to work together at thermo fisher. not long after, along came two daughters. but even as busy parents, dan and michelle always planned a

memorial day 2012 was no exception. how was she that night? happy? >> absolutely. >> reporter: normal? >> uh-huh. normal. >> reporter: the next morning, michelle got up to go to thermo fisher even earlier than usual, to wrap things up before enjoying some "staycation" time with the family. dan stayed at home. he'd already started his week off. >> she wakes me up, kisses me good-bye, says, "i love you," and went to work. >> reporter: and that was the last you saw of her? >> that's the last i saw of her. >> reporter: at 5:53 a.m., a security camera captured michelle's car as it arrived at the parking lot. she stopped by the warehouse's time clock and headed upstairs to her office. about an hour later, a supervisor named ed yuska noticed a big stain on the upstairs hallway carpet. he started looking around the

co-worker, the janitor, david dooley. >> ed was out on the mezzanine just part of the ways, and i was holding the door. and there was just -- he said, there's a dead body laying there. >> reporter: what did it look like? >> i just saw from the knees down. i didn't see the whole thing. but honestly i'm glad i didn't look because i'd never been around anything like that. it was kind of frightening for me. >> reporter: someone inside thermo fisher had killed michelle mockbee. but who? and just as puzzling, why? >> when we come back, lockdown. the hunt for a killer begins. >> the perpetrator could still be there. >> in this giant warehouse, a giant question -- had someone carried out the perfect crime? >> there's actually industrial strength bleach there. rubber gloves everywhere. if you needed to commit a

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>> reporter: tuesday morning after memorial day deputies from the boone county sheriff's department responded to a call of a person down at the thermo fisher warehouse. they rolled up to the front door and were directed upstairs. deputy joe gregory was with them. >> we went over to the victim, assessed that she was dead. there was plastic wrapped around her head.

the back of her hands. >> reporter: the deputies did a tactical search of the area to secure it. >> you definitely have your guns drawn because the perpetrator could still be there. >> reporter: there was no sign of anyone upstairs, but the csi team that arrived soon after found hints of what had happened. a trail of blood in the hallway. a larger blood stain showing smears when lit up, as though someone had attempted a clean-up. the body itself was found in an unused mezzanine area where someone had also stashed michelle's purse and a bag holding employee time cards. >> obviously we felt like the body had been moved. >> reporter: putting the story together was now the job of lead investigators bruce mcvay and everett stahl. tell me a little bit about the building where this takes place. >> thermo fisher scientific is a company that ships medical supplies. kind of like an amazon for medical supplies. it's a large warehouse.

try the size of four football fields. that's ulcer-inducing acreage for an investigator searching for clues. but on the plus side, the warehouse was also a secure facility. detectives say it's pretty hard to get inside if you're not an employee. so you know that your suspect is one of those people in the building? >> that's right. >> reporter: the sheriff's department put thermo fisher on lockdown. no one allowed in or out. deputies took a head count. there were 13 workers on site. detectives began interviewing them one by one. >> i'm sitting actually closer than you and i are sitting to the people that i'm interviewing. and i can see with each person i talk to, there's no blood, there's no evidence that they have been involved in anything. >> reporter: if it's one of these people you're interviewing, they've disguised it well? >> they have. >> reporter: dan mockbee says back at home, he started hearing about an incident at the warehouse. >> they weren't letting people

they were turning them away. >> reporter: dan says he tried calling his wife and other co-workers, but no one was picking up. >> and that's when i started getting nervous. >> reporter: he drove over and was escorted inside by detective stahl. >> everett stahl sat me down at a table and started to ask me a few questions. and i looked at t m and i said "excuse me, officer. my wife works here. i really need to know that she's all right." and that's when he told me that she was deceased. >> his reaction was pretty excruciating to watch. i still had to press on, and i still had to move forward with my looking at him as a possible suspect. >> reporter: while dan mockbee was being questioned, michelle's family was starting to get word from the scene. >> i think i threw the phone down. and i'm just like no, no, this isn't happening. this isn't real.

lockdown at the warehouse was lifted. >> i had each person walk through their day and it seemed like a normal day for everybody. nobody stood out. >> reporter: and then, every thermo fisher employee was sent home. as investigators wrapped up their first major sweep of the building, they were puzzled. >> there was really nothing out of place in this facility. after all ththsearching, the offices, the warehouse, there was just nothing. >> reporter: it didn't help that thermo fisher's shelves seemed perfectly stocked for a criminal. >> this is a company that has tyvek suits, which are what you would see in some murder investigations the detectives wearing, so they don't contaminate scenes. there's actually industrial strength bleach there. >> reporter: that's stronger than any you could buy in a store. >> that's right. rubber gloves everywhere, plastic bags. i mean, if you needed to commit a murder, this place has it.

anything amiss in the entire warehouse? michelle, as head of payroll, had a locked office, and it looked as though someone had tried to break into it. you could tell t tt because? >> there were pry marks on her office door. >> reporter: what was in that office that anybody would want? >> that's the mystery. you know, we talked about money, but there was very little money if any at all. >> reporter: the following morning, thermo fisher employees came back on the job, shell-shocked about the death of their co-worker and jittery about possibly working alongside her killer. at a command center meeting at the sheriff's office, detectives put their heads together and drew up a list of potential suspects, based on the last people to see michelle mockbee alive, or the first to find her dead. ed yuska, the supervisor. >> he was one of the two that found the body. >> reporter: dave dooley, the janitor.

when the body was found. >> reporter: doug tungate, a temp employee, and joe siegert, a warehouse worker. >> doug tungate and joe siegert were the two that saw michelle when she came in. >> reporter: and dan mockbee. >> of course, dan mockbee because he's the husband. >> reporter: a short list of potential suspects for prosecutor linda tally smith. >> i've referred to this case as almost being like the game of clue because you have a very small group and it's a matter of accounting backwards and excluding people. >> reporter: so that's what faced investigators. in which room? with what weapon? and finally, who? coming up -- the questions begin. where was michelle's husband dan the morning of the murder? >> i was sleeping. what a great alibi, right? >> why did you sleep on the

it >> reporter: michelle mockbee's siblings remember her as the big sister who literally couldn't lose. >> she was the contest queen. she won every contest there was for like a two-year run. >> yeah. you name it. she won tvs. flat screen tvs. >> tickets to everything, gift cards, you know, everything. >> super bowl parties. >> lucky. >> she loved it. it made her so happy. and it made her even more happy to share her winnings with other people. >> reporter: but now it was michelle's family that had lost so much. >> we were completely

would want to hurt her. >> reporter: as investigators worked through their list of five initial suspects, one by one, like the prosecutor's game of clue, they started at the most obvious square one, the victim's husband. first couple of days police weren't letting you out of their sight. >> no. >> reporter: were you worried you'd be put in jail? >> absolutely. you know you're as innocent as can be, but there's always a possibility of something. >> reporter: dan was a suspect almost from the gegego. were you guys aware of that? >> i was aware of it. because the detective that came to my house asked me if there was any trouble in their marriage. to which i replied no, not at all. i not for one second thought that dan would be responsible ever. and i don't think any of us did. never. >> reporter: but detectives weren't so sure. they peppered dan with questions. including the big one. when police asked you where you were at the time your wife was killed -- >> i was doing what most

6:00 in the morning if they don't have to get up and go to work. i was sleeping. what a great alibi, right? i'm home in bed. it's not a very good alibi, but it's the only one i had. >> reporter: but exactly where he was sleeping that night raised a red flag for detective stahl. dan volunteered he was downstairs on the couch. was that a sign of trouble in the marriage? >> i said, "well, why'd you sleep on the couch and she sleep in the bed?" and he said, "well, you know, typically the girls sleep with her and i get off of my shift late. by the time i get home, she's already in bed for the next day." so he had just made a habit to sleep in on the couch. it made us want to take a closer look at dan. >> reporter: detectives weren't the only ones interested in the husband. a cadaver dog named little joe had been called in to search outside the warehouse. the dog searched two cars with no success, but then he went up

minivan. >> the dog did have some interest in the back of that vehicle. >> reporter: detectives knew what the dog handler didn't, that the silver minivan belonged to dan mockbee. >> after the dog's up in the vehicle searching, he was finding nothing. there was no findings of human remains or blood. >> reporter: a disappointing dead end. >> investigators have been in and out of this place all day long. >> reporter: but the cadaver dog was then sent over to the mockbee house to join a search in progress. one dan had consented to. >> whatever we asked of dan, he was willing to do. >> reporter: suggesting either that he wasn't guilty or that he was supremely confident he wasn't going to get caught? >> that's right. >> reporter: while dan was feeling the heat, detectives were pulling numbers off michelle's cell phone and tracking down truckers who had delivered to thermo fisher. and of course, stahl and mcvay were taking a hard look at the other four names on their initial suspect list.

to see michelle mockbee alive. one, a warehouse worker named joe siegert, had seemed unusually unemotional about michelle's death when he was first questioned. >> he was one of the ones that made it to the top of the list. >> joe presents himself as a loner. kind of a dry, dark sense of humor and personality.y. >> reporter: the other guy at the time clock that morning was doug tungate. doug was a temporary employee. a newcomer to thermo fisher and an outsider. >> we started looking into his past. saw that he had a couple felonies and some alcohol issues. so we thought, this might be a good guy to look at. >> reporter: then there was david dooley, the janitor. he and his wife janet had cleaned at thermo fisher for years. david was one of the two to find michelle's body. >> detective mcvay interviewed him the day of the homicide, and really nothing that put him to

>> reporter: and last, there was ed yuska. he was the supervisor who called 911 after finding michelle. >> the blood is right by her office door. it looks like she never got in her office. >> reporter: detective stahl found out that ed yuska had some health issues. >> the thought that ed could do all this to michelle, drag her down the hallway and get her to the mezzanine logically to us says ed's not our guy because he physically probably couldn't have done that. >> reporter: which made detectives feel comfortable eliminating ed yuska from their suspect list. and so then there were four. now, how to rule out the next one? the csi team had collected some dna from michelle's body and belongings, but detectives knew it would be months before a forensic lab could report whether any suspect was a match. meanwhile, investigators were starting to scan through a promising new piece of evidence,

warehouse's parking lot. could the clue they needed be somewhere in those thousands of frames of footage? >> there it is. coming up -- >> it was nerve-racking. >> another elimination round. this time the lie detector test. and police soon detected something was up. >> he stops himself at the door and says, no, no, no, i'm not doing this. even if she gets a stain she'll wear it for a week straight. so i use tide to get out those week old stains and downy to get it fresh and soft. and since i'm the one who has to do the laundry, i do what any expert dad would do. i let her play sheriff. i got 20 minutes to life. you are free to go. tide and downy. great on their own

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mother of two michelle mock bee's family -- funeral brought her family some comfort. >> in your mind, you're like,

>> soon red ribbons popped up in honor of michelle. but still couldn't answer the family's questions. >> i feel like we're kind of in limbo. >> we didn't know what this was all about. we didn't know if it was somebody after our family, why they would be, we don't know. >> early on, the investigation had yielded frustratingly few clues on the four remaining lead suspects, but the detective's commander kept up hope. >> i knew once we can get that just one little bit of lead on this investigation it would open, many, many doors. >> hov and his team of detectives were hunting for the next lead, by scanning through hours of security footage of the thermofisher parking lot. >> we had several eyes on this video. >> and stahl decided to turn up the heat on the four, starting with the husband, dan. >> we asked him, would you take

>> you agreed to take the polygraph. >> yes, sir. that's a scary thing. >> because if it goes wrong, all of a sudden there's a case against you. >> right. it was nerve-wracking. i mean, i took a deep breath like this an the guy's like hold on, hold on. you can't do that. you know, it's registering on the sensors. and you're like what if i took a deep breath at the wrong time? >> reporter: but dan told detectives he was eager to be put under the microscope. >> please. do all this stuff so you can figure out that i have nothing to do with this so you can go find whoever did this. >> reporter: dan mockbee passed the polygraph. and this time, the husband didn't do it. and so then there were three. and around that time, major huff and his team spotted something they thought was unusual on the security footage. >> there it is. >> reporter: a vehicle in the parking lot, not entering, but leaving, around the back of the building. >> a bright red, fire engine red, chevy pick-up truck, two-door.

any employee to take. >> reporter: detectives matched the truck to its owner, david dooley, the janitor. >> dave dooley's truck is seen leaving the parking lot at 6:31 that morning. >> reporter: right after the murder? >> that's right. that's right. >> reporter: and detectives could tell from the security tape that dooley had returned to the warehouse around 7:00 a.m., in time to be with ed yuska when he discovered michelle's body. detectives went to the apartment david dooley shared with his wife janet to ask where he went during that half-hour trip. >> they got there at 6:00 -- about 6:30 in the evening. and i kind of figured it would happen just to do a follow-up. >> he told me why he left the building. that he was trying to call his wife. she wouldn't answer the phone. >> my wife does take ambien to help her sleep at night and i went home to check on her, make sure she was okay. >> reporter: his wife, janet, said her husband often worried about her health and would check in on her. >> he drives me crazy with, janet, did you take your medicine? janet, did you take your

that's just the kind of person he is. he's a very sweet guy, a loving guy. >> reporter: but detectives wondered, did he really come home to check on his wife? or to get rid of evidence? >> we started working on a search warrant for the residence and for the truck. >> reporter: you execute those search warrants and you find bloody clothing? >> no bloody clothing. >> reporter: bloody foot prints? >> no. >> reporter: stuff taken from the crime scene that shouldn't be at the house? >> no. >> reporter: some kind of murder weapon? >> no. now, okay, maybe this isn't the guy? >> while they're searching the house, they collected some clothing, and then, just some other odds and ends. but nothing that directly tied michelle mockbee to him. >> reporter: so investigators kept working. they asked two other suspects, joe, the warehouse worker, and doug, the temp employee, to take polygraphs. both men agreed. both passed. two more names scratched off the list.

dooley to take a polygraph. >> and he said, "yeah, i'll take one." >> reporter: but when they brought him down to headquarters -- >> we walk him in, and detective mcvay says, "this is our polygraph operator." and he stops himself at the door and says, no, no, no, i did not agree to that. i did not agree to this. i'm not doing this. >> there was no asking me to do it. it was sit down and do it. i said, well i want to do this with an attorney. >> reporter: dooley didn't take a polygraph at a later time either, but he says that wasn't his fault. >> i did offer to take the polygraph, and they said it's too late. >> reporter: over the next few months, detectives focused in on dooley. significantly for detectives, he was the only one they could ever identify who left the building around the time michelle mockbee was killed. after eliminating other suspects, detectives now believed they'd cracked the case.

county sheriff's department arrested david dooley. >> just very glad he's behind bars. that's where he needs to be. as to why this happened, we have no idea. >> reporter: and then there was one. >> i didn't know what to say. i didn't know what to do. i was never going home. >> reporter: you want to stop a minute? >> it's fine. i knew i was never going home, and i was upset. >> reporter: did you tell them they had the wrong guy? >> yeah, i did. >> reporter: didn't do any good? >> no. >> reporter: but then, the long-awaited results from the dna found at the crime scene came back from the forensic lab. it turned out, david dooley was not a match. and in the local media, janet called for him to be freed. >> we've been telling everyone he didn't do this. nobody deserves to be accused of a crime of this magnitude without something to prove that he did it.

forensic evidence tying david dooley to the crime, prosecutor linda tally smith remained confident, deciding to move forward with the case. in september 2014, david dooley went on trial for murder. >> the truth is, there is no smoking gun. there is no one fingerprint. there's no eyewitness. >> so what did the prosecutor have on david dooley? coming up -- >> our belief has always been that david dooley was in the middle of breaking into her office when she came up the steps and surprised him. >> a break-in? what was he after, and how could he have managed a spotless escape? >> got a clean pair of jeans. clean shoes. >> when "dateline" continues. well the squirrels are back in the attic. mom? your dad won't call an exterminator... can i call you back, mom? he says it's personal this time... if you're a mom,

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it's what you do. >>what took you so long? if you want to save fifteen percent or more on car insurance, you switch to geico. it's what you do. >>you did it, yay! >> reporter: michelle mockbee's favorite color had always been red, and since her death her family has worn red in her honor. you're all wearing your red wristbands today. >> yes. they say, "michelle ann mockbee, in our hearts. 1969-2012." >> reporter: that was your idea? >> yes. just a little something to remember her by. >> reporter: there was a sea of red each day, as michelle's family and supporters flowed

courthouse where david dooley faced trial for her murder. >> on may the 29th of 2012 dan mockbee lost the love of his life, and two little girls lost their mom at the hands of a man who couldn't even keep his story straight from one day to the next. >> reporter: prosecutor linda tally smith began by telling the jurors they wouldn't hear about any smoking gun, but they would hear david dooley's own words which she said had made him the last man standing in the detectives' process of elimination. >> it was through this process that the path kept turning back to one person, the defendant, david dooley. >> reporter: the prosecutor explained that at thermo fisher, warehouse employees used hand-held scanners like this one on the job, and the scanner data could help locate where

of the murder. >> we were able to create, pretty much so, a time record of where everybody was and what they were doing at different points during the morning. >> reporter: jurors heard that almost all the employees were working on the warehouse floor, far away from the upstairs office area where michelle was killed. but where was david dooley? because he was the janitor, he didn't use a scanner. which meant, said the prosecutor, he could have been upstairs near michelle's office during that critical time. david dooley was captured on security video that morning. >> there's the red pickup truck. >> reporter: his red truck was seen leaving the thermo fisher parking lot. >> the defendant actually left the building that day at 6:31 a.m., which was about a half hour after michelle walked into the building. >> reporter: the prosecutor filled in what she believed happened between the time michelle mockbee arrived and when david dooley left. >> she entered the warehouse,

time clocks and headed upstairs into the offices to do her work. >> reporter: the prosecutor showed the jury a photo of fresh pry marks on michelle's office door, which she said indicated an attempted break-in. michelle, you'll remember, had come in earlier than usual that morning. >> our belief has always been that david dooley was in the middle of breaking into her office when she came up the steps and surprised him. and ultimately she was assaulted and restrained. >> reporter: because she was a witness the a crime in progress? >> absolutely. >> reporter: the medical examiner said michelle was bludgeoned to death with something similar to an industrial packing tape gun. but after such a violent attack, why was no blood evidence ever found on david dooley? and why wasn't any of his dna left at the scene? the prosecutor argued that's because the janitor worked every day with cleaning supplies and

perfect for removing evidence of a crime. >> at the time david dooley attacked michelle in that hallway what did he have with him? a rolling crime scene clean-up cart with a trash bag in the middle of it. >> it's my personal belief that when he left that building he took with him a bag containing all of the evidence that was missing from the scene. >> reporter: but you never found any of that stuff? >> never found any of it, no. >> reporter: and once dooley was back at the warehouse, detectives recalled, he looked noticeably fresh. >> this guy's the janitor, but he's really clean. he's got a clean pair of jeans, clean shoes. >> reporter: a cover-up so spotless only a janitor could manage it, said the prosecutor. it was only his story, she said, that was a mess. >> it's very hard for a person to explain how they can't tell the same story twice. and in these circumstances we had four separate statements from him.

twice. >> reporter: detective mcvay testified it was what dooley didn't say in his first interview on the day of the murder that made them take a hard look at him. >> at any time did he tell you that he left that building? >> no, he did not. >> during that first interview, did he tell you that he was one of the people who found michelle mockbee's body? >> no, he did not. >> reporter: mcvay says it was only in dooley's second interview at his apartment that he first mentioned leaving work and going home. >> and i came back here. >> okay. what time did you come back here? >> 6:30. i couldn't get hold of my wife. i came home to make sure she was okay. >> reporter: but detective stahl says dooley's wife janet told stahl something different. >> i ask her specifically if dave ever came home that morning, and she says no. >> reporter: in a later set of his and hers interviews, janet did say david did come home to change a pair of ripped pants. >> he had to come home and, you know, just grab a pair of pants. >> reporter: it sounds like an alibi. except david dooley adamantly

>> i never said that. >> okay. you didn't rip your pants? >> no. >> reporter: however, jurors heard that a warehouse co-worker also remembered dooley talking to him the morning of the murder about ripped pants. >> he had a conversation in which he said that he had to go home and change his clothes because he had ripped his pants. >> reporter: while the prosecutor had focused on david dooley's own words and that suspicious drive away from the murder scene, next the defense was about to point out to the jury all the hard evidence investigators didn't have on david dooley. >> dna evidence, murder weapon, blood evidence, marks on david dooley. >> reporter: and the defense would ask who else might have more of a reason to want michelle dead. coming up -- >> did you have anything to do with the death of michelle mockbee? >> no, i did not. i did not kill her. >> mystery dna.

a husband on the stand. so many questions. and a final bombshell from the

>> >> reporter: jurors had listened for twelve days as prosecutors laid out a very circumstantial case against david dooley. so circumstantial that his defense attorneys chris roach and tom pugh say they sometimes wonder why dooley was even on trial. >> we were trying to figure out exactly what they were saying linked david to this murder at all. the one thing that we could see that was bad for him at that point was just that he went home that day. >> reporter: in court prosecutors had focused on that trip home and, according to

mentioned the trip in his first statement to them. but the defense said because that interview wasn't recorded, what can anyone really know about what was or wasn't said? >> without actually having his interview, now we're going based on one detective's notes saying that he didn't go home that day. >> reporter: david dooley says he knows what he told them. >> i did tell them i went home. i was always very adamant about that. >> reporter: dooley never testified in front of the jury, but he did talk to "dateline" about the case against him and his story that differed from his wife's. you say you went home to check on your wife. they talk to your wife, and she says he never came home. then later, in a separate interview she says he came home because he tore his pants and he came home to change his pants. so what's the truth and why can't you and your wife agree on the same story? >> we do agree that i came home. i have never heard an interview where she said that i did not come home. this was his testimony.

>> as far as i'm concerned, yes. as far as i'm concerned it is not the truth if it cannot be factually proved. >> reporter: did you go home to change your pants? >> no, i did not. >> reporter: why would your wife say that you did? >> i do not know. we've talked about that a couple of times. and the only thing we can come up with is she didn't hear me properly. >> reporter: janet says she's been diagnosed deaf in one ear. on the day of the murder, she says she only saw a pair of ripped pants in the house and thought david said that's why he came home. >> i told them what i thought i heard him say. so a person that cannot hear, they put things together themself through their eyes, and i did. >> reporter: in court, the defense sought to set the record straight for the jury. >> dave didn't change his clothes, and dave didn't change his story. >> reporter: as for that co-worker who also had a story about dooley telling him he ripped his pants? >> why did he wait five to seven days later on to all of a sudden

this? >> reporter: and the defense posed the million dollar question to the jury -- who would want michelle mockbee dead? >> is money a motive? for whom? not david dooley. >> reporter: the jurors heard that the mockbees had around $25,000 in credit card debt. dan said he had no idea. >> i never took care of the finances at all. she wanted to do it, and i was more than happy to let her. >> reporter: the defense then asked dan about the insurance pay-out he received after his wife's death. >> do you recall the amount of that payment? >> mm, 700 grand, something like that. >> we're not saying that we know or believe whether or not dan mockbee killed his wife. but it's about reasonable doubt for our client, david dooley. >> reporter: but more significant to the defense was the unknown dna found at the crime scene.

or on her belongings in at least five different places. >> we heard testimony that there were many unknown dna profiles. could one of these unknown profiles have been the killer? >> reporter: the defense also disputed that thermo fisher was a secure facility and noted that something had set off the warehouse alarm system just three days before the murder. and you think that's significant? >> yeah. yeah, it's significant. that means that someone could have gained access to thermo fisher. >> reporter: after both sides had presented their cases, it was time for closing arguments. now each side would have its last opportunity to speak to the jury. the defense went first. >> no one could think of any reason to kill michelle mockbee. so what motive would david dooley have to kill michelle mockbee? >> reporter: but it was only after the defense had wrapped up its closing that the prosecutor gave her answer to that

of motive for first time. >> i would suggest to you that the evidence is right in that stack of stuff over there. you have time cards, you have invoices, all kept in michelle's office. motive was the time cards? >> yeah. i believe that michelle had actually discovered the fact that he had actually been triple-dipping by clocking himself in, clocking his wife in and getting paid hourly to do a job that they were already being paid a monthly salary to do. >> reporter: that feels like a thin motive. >> sometimes, desperate people do desperate things. >> reporter: the defense attorneys never had a chance to challenge that theory in court, but they say there was zero evidence presented by the prosecution to prove it. >> the thing about it is if there was some evidence, they would have put it in there. >> reporter: what about the theory that you were double-dipping your pay? >> it is not true. >> reporter: you were not double-dipping your pay or triple-dipping or stealing? >> no, we were not. >> reporter: but the jury would

based only on what was presented in court. after deliberating for some 16 hours over two days, word came there was a verdict. >> we the jury find the defendant, david dooley, guilty of murder under instruction number five. >> reporter: guilty of the murder of michelle mockbee. the jury foreperson told "dateline" that in the end david and janet's own statements, which jurors felt were inconsistent, helped them reach their verdict. david dooley was sentenced to life in prison. he says he regrets not taking the stand and has filed for appeal. did you have anything to do with the death of michelle mockbee? >> no, i did not. i did not kill her. >> reporter: you know getting convictions reversed on appeal is a long shot. >> but when you're innocent, it does happen. and it will happen just like it always has. >> reporter: you think you'll be