By Holly Yan, Philip Victor and Susannah Cullinane, CNN

(CNN) -- A "crazed lunatic" turned an outdoor concert into a bloodbath, killing 59 people in what is now the deadliest mass shooting in modern US history.

Thousands of country music fans ran for cover Sunday night as a gunman fired hundreds of bullets from a hotel on the Las Vegas Strip, officials said.

At least 527 people were hurt in the gunfire and ensuing stampede at the Jason Aldean concert, police said.

"About five feet from me, you could see a guy with a bullet wound right in his neck, motionless," witness Taylor Benge said. "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."

The gunman, identified as 64-year-old Stephen Paddock, unleashed a hailstorm of bullets from the 32nd floor of the nearby Mandalay Bay Resort and Casino, Sheriff Joseph Lombardo said Monday.

But why the massacre happened remains a mystery.

"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."

Latest developments

-- 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, Sheriff 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.

-- Police found 10 rifles inside Paddock's hotel room, Lombardo said. "We believe the individual killed himself prior to our entry," the sheriff said.

Festival turns into massacre

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.

The investigation

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.

"We're still just completely befuddled. Dumbstruck," Eric Paddock said outside his home in Orlando, Florida.

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 weapons.

Blood donations needed

With hundreds of victims still hospitalized, officials feared the death toll will keep rising.

The sheriff implored the community to donate blood. And hundreds of Nevadans did exactly that.

Shanda Maloney tweeted a photo while she stood in line at 4:30 a.m.

"This. Is. Vegas. This is our community. These are our people. Thank you to everyone here donating," she tweeted.

Maloney told CNN she also gave transportation to anyone who needed it after the attack.

"I just started picking people up and giving people rides," she said.

Aldean speaks out

Aldean posted a statement on Instagram saying that he and his crew were safe.

"My Thoughts and prayers go out to everyone involved tonight. It hurts my heart that this would happen to anyone who was just coming out to enjoy what should have been a fun night," he wrote.

Country singer Jake Owen, who was on stage with Aldean, said children were among the crowd.

"I saw kids on their parents' shoulders tonight," he said. "This is something they'll never forget."

Trump says he will visit Las Vegas after massacre

10 minutes of gunfire

Rachel De Kerf filmed her escape, starting just after the first shots were fired.

"The gunshots lasted for 10-15 minutes. It didn't stop," she said. "We just ran for our lives."

Her sister, Monique Dumas, said everyone dropped to the ground as as the gunman sprayed bullets.

"It seemed there was a pause in the gunfire, and the people in the yellow shirts were telling the people to 'go, go, go, go,' " she said. But "the gunfire never ended, it seemed like it went on and on and on.

A concertgoer told CNN affiliate KLAS that frantic concertgoers piled on top of each other, trying to get out of the shooter's line of fire.

"My husband and I ran out toward our car, and there were people hiding underneath my car for cover," she said.

"There was a gentleman who was shot and he said, 'Can you help me?' And so I put him in my car, and I had like six people in my car -- people without shoes, running, just to get away."

'Like shooting fish in a barrel'

Audio of the shooting suggested that the shooter had used a military-style weapon, CNN law enforcement analyst James Gagliano said.

"Automatic weapon(s) like that -- had to be numbers of magazines or a very large drum," he said.

"It sounded to me like a belt-fed weapon, a military-style weapon. And then to be shooting down, to use the analogy, it was like shooting fish in a barrel in that space."

MGM Resorts, which owns the hotel the gunman fired from, tweeted its condolences.

As local hospitals rushed to treat hundreds of patents, some relatives were still trying to find their loved ones.

Those looking for friends and family still missing after the attack can call 866-535-5654. Facebook has set up a crisis response page to help people determine whether their loved ones are safe.

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