Las Vegas authorities say they're confident they will find out what set off Stephen Paddock, the gunman in the Sunday night massacre that killed 59 and injured more than 500.
Investigators are counting on help from Paddock's girlfriend, now back in the country.
And they've released body-cam video from police that helps to understand the frantic moments before they found Paddock.
The bodycam video from police shows chaos and confusion, as the attack unfolded.
"Hey, they're shooting right at us, everybody, stay down, stay down. Where is it at?" a voice shouts.
The video also shows police doing whatever they could to help concertgoers escape, and save lives.
"You guys, get down....go that way," an officer says on the footage.
Investigators say Stephen Paddock was meticulous in planning the attack. He set up cameras inside his hotel room and on a service cart outside his door, apparently so he could see if anyone coming to capture him. A hotel security guard who approached his room during the rampage was shot in the leg.
"He fired off and on for somewhere between 9 and 11 minutes," Kevin McMahill of the Las Vegas Metropolitan Police Department said.
Authorities found almost two dozen guns in Paddock's hotel room, along with "bump stock" devices that can allow a rifle to fire continuously, like an automatic weapon.
But what they'd really like to know is what motivated Paddock to plan and execute the deadliest mass shooting in modern U.S. history.
Profilers think there may have been a major trigger in his life, like a great loss or break-up or news of a terminal disease.
And the Las Vegas Review-Journal reports that Paddock was prescribed an anti-anxiety drug this past spring. Records show Paddock purchased the diazepam, also known by its brand name Valium, on June 21.
The paper contends the drug can lead to aggressive behavior, citing drugabuse.com in saying, "Chronic use or abuse of sedatives such as diazepam can also trigger psychotic experiences."
Investigators could learn more soon. Paddock's girlfriend Marilou Danley returned from the Philippines to Los Angeles late Tuesday night. She was met by FBI agents at the airport.
Authorities say Paddock transferred $100,000 to the Philippines in the days before the shooting.
In Las Vegas, more than 500 people are recovering from injuries in the attack. Forty-five were in critical condition, and survivors are still trying to process what happened.
"It still kind of hasn't fully hit me – like, it's kind of starting to – but not fully," said survivor Mikkenna Parry.
"It seems surreal. Like a bad nightmare I'm gonna wake up from. But I just keep seeing it in my mind still and it just haunts me," Sheldon Mack said.
Across Las Vegas, memorials are popping up, as family and friends attempt to honor those who died, and those wounded in this historic tragedy.