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. i don't know how anyone could 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. >> there was a lot of blood, a lot of trauma. >> a mom of two, murdered at work. >> she wakes me up kisses me good-bye that's last of her. >> she got in extra early that morning, just a handful of co-workers in that building. was one of them her killer? >> i 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, coworker to coworker 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. >> who done it? >> sometimes desperate people do desperate things. i'm lester holt and this is "dateline." ♪ >> 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 ant hill of activity as the first employees of the morning

arrived. ♪ >> reporter: 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 her office and i think somebody killed somebody upstairs in her office. >> okay. what makes you think somebody killed somebody? >> she is laying there on the ground and there's blood all over. >> reporter: impossible to believe in this tight-knit workplace where everyone knows everyone but there was michelle mockby 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. >> 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 mockby'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. >> no. >> she was our big sister. >> uh-huh. >> 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 anyone. >> 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 world-wide supplier of laboratory equipment. it was also where she met her husband, dan mockby. >> what was she like? what drew you to her? >> michelle was funny, attractive intelligent. >> reporter: i heard you say that pictures don't do her

justice. >> 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've been thinking about michelle. >> oh, absolutely. i've been working on her for years. >> reporter: he asked michelle to stick with him that night, and she did. and then everything went 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 screwed this up totally. it was terrible. but then i asked her out again and she said yes. >> reporter: maybe you did something right. >> i must have. i don't know. or she felt sorry for me and 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 just 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 date night every other monday, memorial day 2012 was no exception. >> reporter: how was she that night? happy? >> absolutely. >> normal? >> uh-huh. normal. >> reporter: the next morning michelle got up to go to thermo fisher earlier than usual to wrap things up before stay-cation time. >> she wakes me up kisses me good-bye says i love you and went to work. >> reporter: 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 area with help from a coworker the janitor david dooly. >> 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. i had never been anything around that. it was frightening for me. >> reporter: someone inside thermo fisher had killed michelle mockby 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 it to commit a murder this place has it. ♪

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healthing. ♪ 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. there was plastic wrapped around 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 what happened. a larger bloodstain showing smears when lit up as though someone attempted a cleanup. the body itself was found in an unused mezzanine area where someone had also stashed michelle's purse and 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 evert stall. >> reporter: tell me 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. >> reporter: how large? try the size of four football fields. that is ulcer-inducing acreage for an investigators 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 i'm interviewing and

i can see with each person i talk to there's no blood, there's no evidence that they've been involved in anything. >> reporter: so if it's one of these people they're interviewing they disguised it well. >> they have. >> reporter: dan mockbee says he starts hearing about an incident at the warehouse. >> they weren't letting people in the building. they were toning them away. >> reporter: dan says he tried calling his wife and other coworkers but no one was picking up. >> and that's when i started getting nervous. >> reporter: he drove over and was escorted inside by detective stall. >> evert stall sat me down an the table and started requesting me a few questions and i looked at him 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 excruciateing 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. >> reporter: by dusk the 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 the searching, the offices, the warehouse, there was just nothing. >> reporter: it didn't help that thermo fisher shelves seemed perfectly stocked for a criminal. >> this is a company that has tie vex suits which are what you

would see in some murder invests the detectives wearing so they don't contaminate scenes. there's industrial strength bleach there. >> that's stronger than you can buy in a store. >> that's right. rubber gloves everywhere. plastic bags. i mean f you needed it to commit a murder this place has it. >> reporter: the only sign of 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. >> reporter: you could tell that 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 coworker. and jittery about possibly working alongside her killer. at a command center meeting in 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 two that found the body. >> reporter: dave dooly the janitor. >> doug a temp employee and joe seager a warehouse worker. >> doug ton gate and joe seager 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 refer 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 couch and she sleep in the bed?

♪ >> 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 flat screen tvs, tickets to everything gift cards. >> super bowl parties. >> reporter: lucky. >> she loved it. it made her so happy and made it more happy to share her winnings with other people. >> reporter: now it was michelle's family who had lost so much. >> we were completely dumbfounded as to why anyone would want to hurt her. >> reporter: as investigators worked through their list as 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. >> reporter: first couple of days police weren't letting you out of their sight. >> not at all. >> were you worried you would be put in jail. >> absolutely. you know you're as innocent as can be but always a possibility of something. >> reporter: dan was a suspect, almost from the get-go. were you guys aware of that? >> i was aware of it because the detectives they came to my house, asked me if there was any trouble in their marriage to which i replied, no not at all. not for one second thought that dan would be responsible ever. 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 ask you where you were at the time your wife was killed -- >> i was doing what most sensible people are doing at 6:00 a.m. 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 stall. dan volunteers he was downstairs on the couch. was that a sign of trouble in the marriage? >> i said well why did you sleep on the couch and she sleep in the bed? and he said well 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 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 to a third vehicle, a silver mini van. >> the dog did have some interest in the back of that vehicle. >> detectives knew what was the dog handler didn't that the silver mini van belonged to dan

mockbee. >> after the dog is up in the vehicle searching, he was finding nothing. there was no findings of human remains or blood. >> reporter: a disappointing dead end. >> investigators had been in and out of this place all day long. >> reporter: but the cadaver dog was 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 would deliver to thermo fisher. and of course stall and mcvay were taking a hard look at the other four names on their initial suspect list. two men who were the last known to see michelle mockbee alive, one a warehouse worker named joe seager 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. >> reporter: the other guy at the time clock that morning was doug tongate. 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. >> then there was david dooley, the janitor. he and his wife janet has 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 the forefront of anyone's mind. >>. >> reporter: and last there was ed yus ka 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 stall found out that ed yuska had health issues. >> the thought that he could do this to michelle drag her down the hallway and get hoer to the mezzanine logically to us says that zed is not our guy. 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 c zirks 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. security video from the 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 wracking. >> 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. want that one. yea, actually i do. it's mucinex fast-max night time and it's got a nasal decongestant. is that really a thing? it sounds made up. i can't sleep when i'm all stuffy. i take offense to that. i'm not going to argue with a talking ball of mucus. i think you're being a little hasty... he's not with me. mucinex fast max night time. multi-symptom relief plus nasal decongestant. breathe easy. sleep easy. start the relief. ditch the misery. let's end this.

♪ mother of two, michelle mockbee has been murdered at the office. she had gone into work earlier than usual that morning to do the payroll. an hour and a half later, she was dead. police have no witnesses, no murder weapon no fingerprints. but they do have four lead suspects all coworkers. one of them michelle's husband, dan. here again, josh mankiewicz. ♪ >> reporter: michelle mockbee's funeral brought her brokenhearted family some small comfort. >> i remember how many people came to the funeral and just the

endless line of people coming in. in your mind you're just like wow, she touched this many people's lives. >> reporter: soon red ribbons popped up throughout town in honor of michelle. but all the community support in the world still couldn't answer the family's questions -- >> i feel like we're kind of in limbo. we don't know really what to do how to move on just yet. >> 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 didn't know but we didn't know. >> reporter: early on the investigation has yielded frustratingly few clues on the four lead suspects but the detective kept up hope. >> i knew once we could get that one little bit of lead on this investigation, it would open many, many more doors. >> reporter: they were now hunting for that next lead. by scanning through hours of security footage of the thermo fisher parking lot. >> when we had several sets of eyes on this video.

>> reporter: and meanwhile, stall and mcvay decided to turn up the heat on the four. starting with the husband, dan. >> we asked him, would you take a polygraph for us. >> reporter: you agreed to take the polygraph. >> yes, sir. that's a scary thing. >> reporter: 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 and the guy is like hold on hold on. you can't do that. it's registering on the censers 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 micro scope. 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. 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. >> bright red, fire engine red, chevy pickup truck, two-door. that's not the normal route for any employee to take. >> detectives matched the truck to it owner, david dooley the janitor. >> dave dooley's truck is seen living parking lot at 6:31 that morning. >> 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 6:30 in the evening and i kind of figured it would happen just to do a followup. >> 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 vitamins today. that's the kind of person he is. he's a very sweet guy, 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 residence and the truck. >> reporter: you execute those search warrants and you find bloody clothing footprints? >> no. >> stuff taken from the crime scene that shouldn't be at his house? >> no. >> some kind of murder weapon? >> no. >> are you thinking 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 agree, both passed. two more names scratched off the list. detectives had also asked david dooley to take a polygraph. >> and he said yeah i'll take one. >> reporter: but when they brought him down to head quarters -- >> 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 didn't agree to this. i did not agree to this. he said i'm not doing this. >> there was no ask me to do it. it was sit down and do it. and 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 focussed 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 had cracked the case. in september, 2012 the boone 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 you know he didn't do this. nobody deserves to be accused of a crime of this magnitude without something to prove that he did it. >> reporter: but even without forensic evidence tying david dooley to the crime, prosecutor linda tally smith remained confident, deciding to move forward with the case. this past september, david dooley went on trial for murder. >> the truth is there is no smoking gun. there is no one fingerprint. there is no eyewitness. >> reporter: so just 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 such a spotless escape? >> he has a clean pair of jeans clean shoes. >> when "dateline" continues. coughing can really be disruptive.

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eh, you don't want that one. yea, actually i do. it's mucinex fast-max night time

and it's got a nasal decongestant. is that really a thing? it sounds made up. i can't sleep when i'm all stuffy. i take offense to that. i'm not going to argue with a talking ball of mucus. i think you're being a little hasty... he's not with me. mucinex fast max night time. multi-symptom relief plus nasal decongestant. breathe easy. sleep easy. start the relief. ditch the misery. let's end this. ♪ ♪ 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 to 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 into this northern kentucky courthouse. where david dooley faced trial for her murder. >> on may 29th of 2012 dan mockbee lost the love of his life. and two little girls lost their mom. at the hand 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 detective's 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 employees were around the time of the murder. >> we were able to create pretty much 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 that building that day, at 6:31 a.m. which was about half an

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, gathered the time card from the 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 as 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 retrained. >> reporter: because she was a witness to a crime in progress. >> absolutely. >> the medical examiner says 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 everyday with cleaning supplies and plastic bags. 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 cleanup 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 is the janitor but he's really clean. he's got a clean pair of jeans, clean shoes. >> reporter: a coverup so spotless only a john tor 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. and he never told the same story twice. >> r. >> reporter: detective mcvay testified that 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 ahold of my wife. >> reporter: but detective stall says dooley's wife janet told stall something different. >> i asked her specifically if dave ever came home that morning and she says no. >> >> reporter: in a later set

of his and her interviews janet says david did come home to change a pair of ripped pants. >> he had to come home and just grab a pair of pants. >> reporter: it sounds like an alibi, except david dooley adamantly denied doing that. >> i never said that. >> you didn't rip your pants? >> no. >> okay. >> reporter: however jurors heard that a warehouse coworker 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 focussed 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 -- >> reporter: did you have anything to do with the death of michelle mockbee? >> no i did not. i did not kill her. >> mystery dna. a possible intruder. a husband on the stand. so many questions and a final bomb shell from the prosecutor. >> sometimes desperate people do desperate things.

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♪ jurors had listened for 12 days as prosecutors laid out a very circumsubstantial case against david dooley. so circumstantial that his attorneys wonder why dooley was even on trial. >> we're 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 he went home that day. >> reporter: in court, prosecutors had focussed on that trip home and according to detectives that dooley never 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 veried a mat about that. >> dooley never testified in front of jury but he did talk to "dateline" about the case against him and his story that differed from his wives. you said you went home to check on your wife and your wife says he never came home. then later she says 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 i did not come home. this was his testimony. >> the detective. >> yes. as far as i'm concerned, it is not the truth if it cannot be

factual proved. >> reporter: did you go home to change your pants? >> no, i did not. >> why would your wife say you did? >> i do not know. we've talked about that a couple of times. 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 themselves 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 coworker who also had a story about dooley telling him he ripped his pants? >> why did he wait five to seven days later on to recall oh yeah david said this. >> reporter: and the defense

posed the million-dollar question to the jury who would want michelle mockbee dead? >> was money motive? for whom? not david dooley. >> reporter: the jurors herd that the mockbee 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 payout he received after his wife's death. >> do you recall the amount of that payment? >> 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. dna was found on michelle's body 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 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. 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 question. laying out her theory of motive for the first time.

>> i would suggest to you that the evidence is right in that stack of stuff over there. we have time cards, you have invoices all kept in michelle's office. >> reporter: you think the motive was the time cards. >> yes. 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 there were already being paid a monthly salary to do. >> reporter: that feels like a thin motive. >> sometimes desperate people do desperate things. >> reporter: 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 have to render its decision 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 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 he plans to 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 a free man one day? >> yes, i do.

>> reporter: michelle's family believes strongly that the jury got it right, though the aftermath is bittersweet. >> nothing is going to bring michelle back. there is no justice that is good enough. >> reporter: now their focus moving forward is on giving love and support to dan and the girls. what did your daughters say to you about this. >> neither one of them want to talk about it to this day because they're just too sad. >> reporter: dan is hoping the two young women he is raising will carry the spirit of the one he lost. >> we talked about mom and things she liked to do. she just exuded personality and fun and happiness and that's what michelle was. ♪ that's all for now. i'm lester holt.

thanks for joining us. this sunday, outpourings of grief at the funeral of a slain new york city police officer. >> when an assassin's bullet targeted two police officers, it targeted this city. and it touched the soul of the entire nation. >> what can be done to alleviate tensions between the black community and police in the country. i will be joined by william bratton. with the stock market at record levels how a booming economy could change politics next year and in 2016. why hollywood and comedy loves washington. >> for those of you climbing to the top of the food chain, there can be no mercy. >> the public may be fed up with politicians, but the entertainment industry just can't get enough. >> we're living on the co