By Holly Yan, Susannah Cullinane and Philip Victor, CNN

(CNN) -- Thousands of country music fans ran for cover as a gunman fired on them from a hotel on the Las Vegas Strip -- a mass shooting that is now the deadliest in modern US history.

More than 50 people were killed Sunday night when the shooter fired hundreds of bullets into the crowd, police said.

Over 400 others were rushed to hospitals after the mass shooting and ensuing stampede at the outdoor Jason Aldean concert.

The gunman, 64-year-old Stephen Paddock, unleashed a hailstorm of bullets from the 32nd floor of the nearby Mandalay Bay hotel, Las Vegas Metropolitan Police Sheriff Joseph Lombardo said.

Inside his room, authorities found a cache of weapons, including 10 rifles, the sheriff said.

"We believe the individual killed himself prior to our entry," Lombardo said.

Festival turns into massacre

The massacre started around 10:08 p.m. Sunday (1:08 a.m. ET Monday) at the Route 91 Harvest festival, Lombardo said.

Taylor Benge was enjoying the concert with his sister when he heard a relentless onslaught of "200 to 300" bullets.

"My sister, being as noble as she is, threw herself on top of me and said, "I love you, Taylor," the 21-year-old said.

"Even after an hour and 30 minutes, I didn't know if I was safe."

Police said they believe Paddock acted alone. "Right now, we believe it's a sole actor, a lone-wolf-type actor," the sheriff said.

But why the massacre happened remains a mystery.

The investigation

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 described as a "person of interest" after the attack is 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 responsible for the deadliest mass shooting in modern US history.

"He was my brother, and it's like an asteroid fell out of the sky," Eric Paddock told CNN outside his home in Orlando, Florida.

The last time he 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 before dawn, throngs of donors had already lined up outside a blood bank.

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 says.

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. #heartbroken#stopthehate," he wrote.

Two Las Vegas police officers are being treated at a local hospital for injuries during the shooting, Lombardo said. One is in critical condition, and the other had minor injuries.

"Pray for Las Vegas," Las Vegas Mayor Carolyn Goodman tweeted. "Thank you to all our first responders out there now."

Nevada Gov. Brian Sandoval said on Twitter that a "tragic & heinous act of violence has shaken the #Nevada family" and offered prayers to all those affected by "this act of cowardice."

'We just ran for our lives'

Rachel De Kerf filmed her escape from the venue using her cell phone, starting just after the first shots were fired.

She described ongoing gunfire, and played out the video she had recorded during which more than five minutes of gunfire were intermittently audible.

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

De Kerf's sister, Monique Dumas, said that everyone instantly crouched when they heard the shots.

"The band was rushed off the stage, the floodlights came on the crowd, and you see on the right hand of the stage the person who was injured, so they're calling for medics, calling for security, then there was gunfire again," Dumas said.

'Go, go, go'

SiriusXM Country radio host Storme Warren was on the side of the stage as Aldean was performing when the shots rang out.

"I thought it was fireworks going off and maybe it mistriggered, and then it happened again. And when it happened the third time, we knew something was wrong," Warren said.

"The shells were hitting the deck of the stage when I was on it," he said, adding that he could still hear the shells as he went under the stage for protection.

"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' ... the gunfire never ended, it seemed like it went on and on and on," Dumas said.

A concertgoer told CNN affiliate KLAS that everybody was lying on top of each other trying to get out of the shooter's way.

"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, 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," Gagliano said.

MGM Resorts, which owns the Mandalay Bay, tweeted its condolences.

"Law enforcement and emergency personnel responded quickly to the incident a secured the scene," it said in a statement. "Law enforcement requested that we put hotels in the vicinity on lockdown to ensure guest safety. We will provide more information as it becomes available."

Facebook has set up a crisis response page to help people determine whether their loved ones are safe.