By Saeed Ahmed and Tiara Chiaramonte, CNN

(CNN) -- At some point Thursday, Stephen Paddock checked into the Mandalay Bay Resort and Casino on the Las Vegas Strip.

He'd driven 80 miles from his home in Mesquite, on the Nevada-Arizona border, for ... what, exactly, no one knew.

His dad had once been on the FBI's most-wanted list. But he was a 64-year-old retired accountant with no known criminal record.

And yet, here he was, holed up in room 32135 with 23 weapons, multiple rifles -- some with scopes on them -- and a cache of ammunition.

Across the street, a three-day country music festival would kick off that weekend. The gilded windows of his 32nd-floor room would offer him a good view.

He waited.

The lineup for Route 91 Harvest was a who's who of country music talent: Eric Church. Sam Hunt. Big & Rich. All weekend long, fans packed the sold-out festival, drinking, partying and singing along.

Now it was 10 p.m. Sunday and the 22,000 concertgoers who remained were there to end their jamboree with Jason Aldean, the final act.

Up on the 32nd floor, Paddock smashed two windows in his hotel room -- one in the front and one on the corner. He picked up a gun, took aim at the fans below, and opened fire. Bullets rained down.

For a few seconds, no one seemed to react. The intro riff to Aldean's "When She Says Baby" had just ended when the sound of automatic gunfire filled the air. It wasn't until the third line -- "Some days I'd rather be a no show" -- that Aldean realized something frightening was happening. He darted to the back of the stage.

For the next 10 to 15 minutes, it was full-blown panic for the 22,000 attendees. They scrambled for cover and hid under bleachers. They jumped over fences and shielded loved ones with their bodies.

People flattened on the grass. And every time there was a temporary break in the gunfire, they'd get up and sprint for cover. Again and again and again.

It felt like eternity. But a SWAT unit swiftly and methodically worked its way up the Mandalay Bay, floor by floor. It cleared out the 30th floor, the 31st, and headed to the 32nd. Paddock shot through the doorway, firing at officers. They stopped. They were about to blow open the door.

They were too late. Paddock had killed himself, leaving behind a whole host of questions. Chief among them: Why?

ISIS was quick to claim credit -- but officials aren't convinced.

"I can't go into the mind of a psychopath," Las Vegas Sheriff Joe Lombardo said. "We have to establish what his motivation was first."

Slowly, the names of the victims are surfacing. Victims such as Jenny Parks, a kindergarten teacher. Or Sonny Melton, a registered nurse, who was at the show with his wife. She survived. He didn't.

At least 59 dead. Some 527 injured. The deadliest mass shooting in modern US history. Yet another addition to a sad and growing list: Pulse, Virginia Tech. Sandy Hook.

Those who were fortunate enough to make it out alive may never be quite the same.

"This is all other people's blood," said one woman, pointing to her legs. "This is all blood splatter from people being dead and us having to be on top of them basically to get out."

The blood will wash away. The psychic scars may not.