>> there's that son of a [ bleep ] jack despain. >> did you think jack was somehow responsible?oh, yeah. marc had told multiple people if anything ever happens to me you'll look at my dad. >> 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 divides a family. >> oh, my god. could this have possibly happened? >> but was that the motive for murder? was it even true? when "dateline" continues. ♪ ♪ with so much great entertainment out there... wouldn't it be easier if you could find what you want, all in one place? my favorites. get xfinity streamsaver with netflix, apple tv+, and peacock included, for only $15 a month. ♪♪ >>> it's an unpleasant thing to think about. parents murdering their own children. but it happens. it's 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 t