Vegas gunman's motivation remains a mystery weeks after deadly attack: Part 3

Stephen Paddock, the high-rolling gambler who stockpiled an arsenal in his Mandalay Bay hotel room, has befuddled authorities investigating what drove him to commit the worst mass shooting in modern American history.
6:38 | 10/28/17

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Transcript for Vegas gunman's motivation remains a mystery weeks after deadly attack: Part 3
I can't sit and dwell because that's when it starts to kick in. The fear or of it maybe happening again. Michael green field was working as a stage hand that night. That sound just keeps ringing in my head. It's like a CD on loop. You keep hearing that sound over, and over again. It's hard to go away. I don't want to say I have PTSD, or anything, but there's a part of me that it feels like I didn't do enough. That gets me the most. Reporter: He may look big and tough, but the 37-year old Greenfield is devastated over what happened, still trying to understand how Stephen paddock could kill so easily. I still want to know more about paddock, and his motivation, and why, and why so many innocent people. Reporter: It's a question that authorities have not been able to answer so far, just as Stephen paddock would have liked it. He loved the mystery because he was a mysterious guy. Reporter: Former FBI profiler Brad Garrett, now an ABC news consultant, took us through the factors in paddock's background that are now being examined. A criminal father. An addition to gambling. A fascination with guns. A twisted loner, tortured with delusions. Next week, portions of paddock's brain will be sent to doctors at Stanford university who will examine it for any brain disorders. There is a suggestion that in brains of very violent people, their brains are a little different Reporter: When you look at the profile of mass shooters, he doesn't seem to fit it, does he? No. He's an older guy, made a lot of money, successful at gambling, successful at real estate. People may not like him, but they probably respect him because of what he's done, but adding to the mystery, there were some people who did actually like him, and did not think he was crazy. He was good to me. He was kind. Reporter: Lisa Crawford worked for paddock when he owned an apartment complex in Texas. The man that I knew, that I thought was my friend, that I thought was a friend of the family. I feel so much shame, I'm embarrassed. Reporter: One thing that paddock was not, was part of ISIS. Even though the terror group tried to claim the 64-year old multi-millionaire was one of theirs. When that came out, I just actually laughed. He wouldn't care enough to join ISIS cause he doesn't really care about anybody else but himself. Reporter: Paddock's only real passion seems to have been gambling. A high roller treated like a king by the casinos. Obsessed with video poker. Making millions at it. He gambled for years and did it mathematically. He was in the poker area at the mandalay bay hotel playing into the early morning hours. He won several big jackpots. Court records show six years earlier paddock did have a encounter at the cosmopolitan hotel when cameras caught a moment he slipped on the floor of the lobby. Paddock sued for damages claiming he tore his ham string but they say he was probably drunk as he was when he showed up to testify. He was carrying a beer in a paper bag, wearing shorts, had very cheap flip-flops on, kind of unkempt looking. Reporter: In his testimony, paddock said he suffered from anxiety, didn't like being out in the sun, and was prescribed valium. It was perhaps an early indication of what investigators now believe was paddock's slow descent into madness. The combination of age, deterioration, and the idea that he's just decided he's just mad at the world that I really want big time attention, and boy does he have it. Reporter: Just as paddock's father did in the 1970's, when he was one of the FBI's ten most wanted fugitives. Young Steven was seven years old when his father, Benjamin Hoskins paddock, was arrested for bank robbery. What's similar between paddock and his father? Anti-social personality, tends to be a manipulator, tends to get whatever they want, tends to blame everybody else for their issues. Reporter: After being released from prison the father opened up a bingo parlor claiming it was part of a church. You guys don't print anything, you don't tell anything but lies, you have never been on my side, forget it. Reporter: One prominent theory now among law enforcement is that paddock wanted to somehow outdo his fugitive father. So is that part of it? Does he want to be more famous than Benjamin? Reporter: On the day of the shooting, paddock made five separate trips back and forth between his home in Mesquite, and the mandalay bay hotel, 70 miles away. Presumably shuttling his weapons, setting up his 32nd floor sniper's nest. He left behind no social media footprint, no manifesto, no suicide note and the hard drives on his computers are missing. I think this is going to boil down to, I want attention, and I want to feel powerful. And that's the motivation to kill all those people? Well that's the motivation for every mass shooter. So that's just sick? That's just sick. Reporter: Tonight, the body of Stephen paddock is being held at a secret location. No funeral home has reportedly been willing to handle the arrangements for a man whose evil acts defy any satisfying explanation. Why? If any of his family members, or his friends knew him so well, why? Somebody knows something, and the plot thickens.

This transcript has been automatically generated and may not be 100% accurate.

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