>> there's that son of a bitch, jack despain. >> reporter: did you think jack was somehow responsibletiple people, "if anything ever happens to me, y'all look at my dad." >> reporter: so, of course, detective brooks invited jack down to the police station. just maybe this mystery would have a quick, if very disturbing, solution. >>> coming up -- a young girl makes a troubling accusation that defines a family -- >> oh my god. could this have possibly happened? >> but was that the motive for murder? was it even true? when "dateline" continues. ♪ >> reporter: it's an unpleasant thing to think about. parents murdering their own children. but it happens. is an old story, frankly. some awful glitch, perhaps, in the human recipe. had it happened here, in jonesboro? suspicion fell quickly on marc's dad, jack despain. michelle's mom heard about it when she called to tell a relative that marc was dead. >> somebody's murdered marc. and he was like, "oh my god." he said, "have they found jack despain?" >> reporter: he thought right away it was jack. >> uh-huh. >> reporter: because anyone close to

jack despain. >> reporter: did you think jack was somehow responsible? >> oh, yeah. marc had told multiple people, "if anything ever happens to me, y'all look at my dad." >> reporter: so, of course, detective brooks invited jack down to the police station. just maybe this mymyery would have a quick, if very disturbing, solution. >>> coming up -- a young girl makes a troubling accusation that defines a family -- could this have possibly happened? >>>>ut was that the motive for mumuer? when "dateline" continues. >> reporter: it's an unpleasant thing to think about. parents murdering their own children. but it happens. is an old story, frankly. some awful glitch, perhaps, in the human recipe. had it happened here, in jonesboro? suspicion fell quickly on marc's dad, jack despain. michelle's mom heard about it when she called to tell a relative that marc was dead. >> somebody's murdered marc. and he was like, "oh my god." despain?" >> reporter: he thought right away it was jack. >> uh-huh. >> reporter: because anyone close to jack and his family knew, a poison far more po

it was all about jack despain's hatred toward me. >> reporter: you seriously believe that?>> reporter: michelle, out on bail, waited for trial. her mother, kathy defended her around town. did you ever let yourself think she was involved? >> the daughter i know? the daughter i helped raise? no. >> reporter: meantime, michelle's attorneys, ray nickle and bill stanley, took a closer look at those recovered text messages. they filed a motion arguing that investigators didn't obtain them properly. >> they didn't get a new search warrant every time they searched the phone. we're talking about, i think 15 searches. >> yeah. >> of michelle and carl's phones. and they didn't have 15 search warrants. >> reporter: their arguments about improper search warrants may have had some traction. with the trial looming, the prosecutor was worried. >> we're at the high stakes poker game where it's all or nothing when that judge rules the next day on whether to admit the evidence or not to admit it. >> reporter: the prosecutor agreed to start talking about a plea deal. the defense played it toug

it was all about jack despain's hatred toward me. >> reporter: you seriously believe that? >> reporter: michelle, out on bail, waited for trial. her mother, kathy defended her around town. did you ever let yourself think she was involved? >> the daughter i know? the daughter i helped raise? no. >> reporter: meantime, michelle's attorneys, ray nickle and bill stanley, took a closer look at those recovered text messages. they filed a motion arguing that investigators didn't obtain them properly. >> they didn't get a new search warrant every time they searched the phone. we're talking about, i think 15 searches. >> yeah. >> of michelle and carl's phones. and they didn't have 15 search warrants. >> reporter: their arguments about improper search warrants may have had some traction. with the trial looming, the prosecutor was worried. >> we're at the high stakes poker game where it's all or nothing when that judge rules the evidence or not to admit it. >> reporter: the prosecutor agreed to start talking about a plea deal. the defense played it tough, said michelle was only willin