right. reporter: ten minutes before he left with cooper? right. we were able to determine that he had been messaging at least six different women throughout that day, sending pictures of himself, asking for photographs. reporter: prosecutors believed ross harris was in the grip of an escalating obsession with sex. and they came to an awful conclusion. that ross wanted so badly to be rid of the constraints of family and fatherhood, he decided to kill his own child. does being a creep with women make him a child killer? absolutely not. in this case, though, it showed us the extent to which he was living the other life. it showed us how it had overtaken in his life what was important to him. reporter: so they threw the book at him. ross was indicted for malice murder, the first person ever to be accused of using a hot car to intentionally kill a child. stunning and damning new details emerge. reporter: the case became a media sensation with more revelations. investigator
and a double life. reporter: chuck boring, the lead prosecutor, told the jury why ross story had never added up. that chick-fil-a video showed cooper wide awake. that drive that was too short to forget a toddler. the car seat was just inches behind ross. and at ross office there was a photo of cooper and a gift from cooper. he even sent a text to leanna in the afternoon about cooper. when you getting my buddy, it said. none of that jogged his memory? there was just cue after cue after cue that if he had actually forgotten would have triggered something. reporter: and then the prosecution showed what it believed was powerful evidence that ross had to have known that cooper was still in the backseat. that video of ross returning to the car. why would ross harris return to the scene of the crime?
there s his toys, his pictures. it s not real if she doesn t go inside. reporter: katie says leanna was numb without her child and her husband. did she go through that emotion at any point, i m angry at you? how could you do this? i don t know that she was ever angry at him. reporter: because you go through that scenario as a parent, what if my spouse did this? would i be able to understand? but she s such a composed woman. and her faith, i don t think, would let her be angry at him for something that he did not do intentionally. reporter: as cooper s family began to grieve, detectives started building their case against ross harris. and what they found convinced them this was no simple case of negligence. they believed this was premeditated murder. coming up did he lie in. he absolutely lied. there were a whole lot of
reporter: but what about a those reminders during the day? shouldn t one of those have triggered his mind to say, wait. cooper. where s cooper? i think that s a fair question to ask. if ross had forgotten that he had a son, seeing a picture of cooper would have reminded him that he had a son. he didn t forget he had a son. once ross slipped into his work day, he had every reason to believe that cooper was at the day care. reporter: as for those witnesses who said ross seemed detached, they didn t know him, the defense argued. so they called people who did, like his friend billy. in my opinion, he was just incredibly loving and doting. reporter: a colleague from home depot. when cooper was born it s all
he said it while he was talking to leanna that, you know, he was dreading how cooper was going to look when he got him out. reporter: almost like he knew what was there. exactly. he knew what was going to be found. reporter: they were also struck by how calm leanna seemed to be. and by this surprising question. did you say too much? reporter: detectives wondered what she meant. they came back into the room and told ross he wasn t going home. he s going to jail. he s being charged. reporter: investigators thought they had enough to prove ross had been negligent by leaving his son in the car. his negligence caused the death of your son. reporter: and their investigation was just beginning. later that night, when katie reached leanna on the phone, she told her she d been sitting outside her home on the sidewalk. and she s told me that, you know, as long as she sat outside, she could pretend like this wasn t happening. because once she goes inside, you know, there s