. >> reporter: a short list of potential suspects for prosecutor linda tally smith. >> i refer to thist 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 inve

>> reporter: but even without forensic evidence tying david dooley to the crime, prosecutor linda tally smithned 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. with a record breaking fifty million votes, your superstar is... that's why there's delsym. delsym helps silence coughs for a full 12 hours with an advanced time release formula for all day or all night relief. up to twice as long as other cough liquids. so the only sounds you'll hear are the ones you want to hear. bianca! (cheering) delsym. silence is relief. and now