But what they couldn't explain is why the man who had never faced any notable criminal charges did it. There was no known motive late Monday.
Even Paddock's brother had no answers.
"We're still just completely befuddled. Dumbstruck," Eric Paddock said in Orlando, Florida.
-- A team of six officers spoke with security at the Mandalay Bay Resort and Casino, where Paddock was staying, and searched the hotel floor-by-floor Sunday night before they found Paddock's room, Clark County Sheriff Joseph Lombardo told reporters.
-- Paddock, 64, fired at the officers through the door, Lombardo said. A SWAT team broke down the door, but Paddock had already killed himself, Lombardo said.
-- Authorities recovered 23 guns from Paddock's room, said Clark County NV Assistant Sheriff Todd Fasulo. Lombardo said several of the rifles had scopes of them.
--Another 19 firearms, along with explosives and several thousand rounds of ammunition in Paddock's Mesquite, Nevada, home. The gunman apparently had smashed out two windows to increase his range of targets.
-- The sheriff said a SWAT team was standing by at a house in northern Nevada. A law enforcement official confirmed the FBI is present in Reno.
-- Sandra Casey, a special education teacher in Manhattan Beach, California, was killed, the Manhattan Beach Unified School District said. "We lost a spectacular teacher who devoted her life to helping some of our most needy students," school board President Jennifer Cochran said.
-- Sonny Melton also was identified
as among the dead. His employer, Henry County Medical Center in Paris, Tennessee, said Melton was a registered nurse. His wife survived the shooting.
-- Police had no prior knowledge of the gunman before the attack, Lombardo said. "I don't know how it could have been prevented," he said.
-- Paddock bought multiple firearms in the past, but investigators believe the firearms were purchased legally, a law enforcement official said. The official said initial reports suggest at least one rifle was altered to function as an automatic weapon.
-- Chris Michel, owner of Dixie GunWorx, in St. George, Utah, recalled selling Paddock a shotgun earlier this year, CNN affiliate KTVX
reported. "He talked about how he just moved closer to where we are," Michel said of Paddock. "He said he was visiting local firearms shops." Paddock lived in Mesquite, Nevada, about 35 miles from St. George.
'Everyone's dying around me'
Witnesses described the horror that unfolded.
Taylor Benge said he "could see a guy with a bullet wound right in his neck, motionless," several feet away. "From there on ... people just started dropping like flies."
Alexandria Cheplak, 25, called her father as she ran from the bullets.
"Everyone's dying around me," Jon Cheplak recalled her saying. "Everyone's dying. They shot my friend ... I've got to get out of here."
Police said Paddock, unleashed a hailstorm of bullets from the 32nd floor of the resort, Lombardo said Monday.
Authorities are still piecing together a motive.
"I can't get into the mind of a psychopath," the sheriff said.
Mayor Carolyn Goodman described the gunman as "a crazed lunatic full of hate."
The massacre started about 10:08 p.m. Sunday (1:08 a.m. ET Monday) at the Route 91 Harvest festival
, Lombardo said.
Benge was enjoying the concert with his sister when he heard a relentless onslaught of at least "200 to 300" bullets.
"The firing went on forever," witness Kimberly Chilcote said.
She and her husband ran in short bursts -- sprinting and hiding to avoid being targeted.
"There were purses and shoes everywhere ... there were bodies and blood," she said. "We just kept running."
Benge lauded the heroics of his sister, who "threw herself on top of me and said, 'I love you, Taylor,'" he said.
"Even after an hour and 30 minutes, I didn't know if I was safe."
Witness Bryan Hopkins said he survived by jumping into a walk-in freezer at the Mandalay Bay hotel.
"There must have been, I don't know, 23 to 30 of us inside this freezer," he said.
Corrine Lomas recalled the heroism of fellow concertgoers, risking their lives to save others.
"A lot of really good people (were) holding people's wounds shut, trying to help them while everybody was just ducked down," she said.
Police said they believe Paddock acted alone. "Right now, we believe it's a sole actor, a lone-wolf-type actor," the sheriff said.
So far, the massacre has no known link to overseas terrorism or terror groups, a US official with knowledge of the case said.
And a woman who was described as a "person of interest" after the attack is now not believed to be involved in the shooting, police said in a statement.
"Marilou Danley is no longer being sought out as a person of interest," the Las Vegas Metropolitan Police Department said. "LVMPD detectives have made contact with her and do not believe she is involved with the shooting on the strip."
The gunman's brother, Eric Paddock, said he was stunned to learn Stephen was believed responsible.
He described his brother, a retired accountant, as "a wealthy guy. He liked to play video poker. He went on cruises."
The last time Eric Paddock spoke to his brother was when Stephen texted him, asking how their mother was doing after losing power from Hurricane Irma.
Eric Paddock said he knew his brother owned a few handguns and maybe one long rifle, but said he did not know of any automatic weapon.
Blood donations needed
With hundreds of victims still hospitalized, officials feared the death toll will rise.
The sheriff implored the community to donate blood. And hundreds of Nevadans did exactly that.