Transcript for Insight into the mind of Las Vegas gunman Stephen Paddock

There is also breaking news tonight involving the Las Vegas massacre. Moments ago, we learned that the hero security guard who showed up first on that hotel floor and was shot through that door, well, now we've learned the massacre had not even begun yet. Authorities say afterward the gunman started to fire, and tonight what we now know about the note left behind. Here's ABC's senior justice correspondent peel area Thomas. Reporter: Tonight, with no clear motive or hard evidence of accomplices, ABC news has learned some investigators increasingly suspect Stephen paddock had a severe mental illness. Comprehensive picture is being drawn as to the suspect's mental state and currently, we do not believe there is one particular event in the suspect's life for us to key on. Reporter: Details now emerging about the note with numbers on it, found in paddock's room. Detailed calculations of height and wind velocity to maximize the death count. He had written the distance, the elevation he was on, the drop of what his bullet was going to be for the crowd. Reporter: And tonight, we've learned that that hero security guard was shot before the massacre happened. Arriving on the scene after an alarm for a door ajar went off in another room. The gunman firing 200 guards at the guard through the door. What we have learned is Mr. Campos was encountered by the suspect prior to his shooting toe the outside world. Reporter: Pad ddock killing himself after shooting into the crowd for 11 minutes. That's when the police arrived. 12 minutes is a long time. Reporter: This friend, who worked with him for six years, is baffled by paddock's final act of terror. He was kind. And I don't know what happened to him. I don't know. Pierre Thomas with us live tonight. And Pierre, there had also been reporting that some of the fuel tanks at the airport, not far from the concert site, had been hit and we're learning more about that tonight, as well? Reporter: David, the sheriff said he may have been targeting those fuel tanks and that he had personal protection in the room. The sheriff said he suspects he had an escape plan. David? Pierre Thomas with us live in Washington. Pierre, thank you.

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