ORLANDO, Florida -- An American-born man who'd pledged allegiance to ISIS gunned down 50 people at a gay nightclub in Orlando -- the deadliest mass shooting in the United States and the nation's worst terror attack since 9/11, authorities said Sunday.
- The gunman was Omar Mateen of Fort Pierce, Florida, a law enforcement source told CNN.
- Mateen called 911 around the time of the attack to pledge allegiance to ISIS and mentioned the Boston bombers, according to a U.S. official.
- Orlando police shot and killed Mateen.
Mateen carried an assault rifle and a pistol into the packed Pulse club about 2 a.m. Friday and started shooting, killing 50 people and wounding at least 53, police said. After a standoff of about three hours, police crashed into the building with an armored vehicle and killed Mateen.
"It appears he was organized and well-prepared," Orlando Police Chief John Mina said early Sunday. Authorities have not described finding any accomplices.
There has been no claim of responsibility for the attack on jihadi forums, but ISIS sympathizers have reacted by praising the attack on pro-Islamic State forums.
"We know enough to say this was an act of terror and act of hate," President Obama said in an address to the nation from the White House.
While the violence could have hit any American community, "this is an especially heartbreaking day for our friends who are lesbian, gay, bisexual or transgender," he said.
People inside the cavernous nightclub described a scene of panic made more confusing by the loud music and darkness.
Christopher Hansen said he was getting a drink at the bar about 2 a.m. when he "just saw bodies going down." He heard gunshots, "just one after another after another."
The gunshots went on for so long that the shooting "could have lasted a whole song," he said.
Ricardo Negron Almodovar escaped the club and posted this description on his Facebook page: "People on the dance floor and bar got down on the floor and some of us who were near the bar and back exit managed to go out through the outdoor area and just ran."
Orlando Mayor Buddy Dyer said he had declared a state of emergency for the city. Florida Governor Rick Scott declared a state of emergency for Orange County, where the attack occurred.
Before Sunday, the deadliest shootings in U.S. history were at Virginia Tech in 2007 and Sandy Hook Elementary School in 2012, with 32 and 27 killed.
Helmet saves officer
Law enforcement sources had told CNN that the suspect had possible explosive devices strapped to his body and in his vehicle, but a U.S. official said later that no explosives were found.
Mina said authorities were called to the club. After a shootout with the gunman around 2 a.m. ET, the shooter ran back inside the club and took people hostage.
People inside the club were communicating on their phones with law enforcement between that time and around 5 a.m., when authorities used an armored vehicle to break down the door of the building. The club is a vast, open space that was hosting more than 300 patrons late Saturday and into Sunday morning.
One officer suffered an eye injury when a bullet struck his Kevlar helmet, said Danny Banks, special agent in charge of the Florida Department of Law Enforcement's Orlando bureau. The helmet saved the officer's life, Banks said.
The shooter, Omar Saddiqui Mateen, is 29 and from Fort Pierce, about 120 miles southeast of Orlando, two law enforcement officials told CNN. He had been trained as a security guard, CNN has learned.
Two officials tell CNN that the FBI had investigated Mateen at some point for possibly having ties to or sympathizing with Islamic extremism. A law enforcement official said there were two cases opened involving Mateen but the probes didn't result in enough evidence to charge him with anything.
In the past two weeks Mateen legally purchased a Glock pistol, found at the shooting scene, from a St. Lucie County area gun store, a law enforcement official said.
Investigators have talked to Mateen's family, who indicated he had expressed anti-gay feelings.
Orlando authorities said they consider the violence an act of domestic terror. The FBI is involved. While investigators are exploring all angles, they "have suggestions the individual has leanings towards (Islamic terrorism), but right now we can't say definitely," said Ron Hopper, assistant special agent in charge of the FBI's Orlando bureau.
Authorities are also looking into the possibility the attack was a hate crime, a law enforcement source told CNN.
Also on Sunday, authorities in West Hollywood, California, took a suspect into custody after discovering an arsenal in his car that included explosive powder, assault weapons and a camouflage outfit, CNN affiliate KABC reported, citing unnamed authorities. The suspect told authorities he was there for the Los Angeles Pride festival, which is underway this weekend.
There was no indication that the arrest was related to the Orlando attack.
Shooting, hostage situation
Video from CNN affiliate WKMG captured clubgoers carrying injured people from the club.
When the shots erupted, Hansen hit the ground, crawling on his elbows and knees, before he spotted a man who had been shot.
"I took my bandana off and shoved it in the hole in his back," Hansen said, adding that he saw another woman who appeared to be shot in the arm.
Luis Burbano was with his best friend inside the club when they realized the pop, pop, pop they were hearing wasn't the music but gunfire.
It was getting, "closer and louder and louder," he told CNN's Jake Tapper. He and his friend noticed a 10-second break in the shooting, so they managed to make a break for the door.
"We tried to save ourselves and as many people as we could to make it out of there," he said.
Once outside he saw a young man collapse in front of him. "I grabbed him not realizing that his forearm was split in two," Burbano said.
He acted fast by ripping off his shirt and tightening it around the man's arm to try to slow the bleeding.
Burbano spoke with CNN hours after talking with police and getting home. He was still very shaken.
'Just bang, bang, bang!'
Pulse describes itself as "the hottest gay bar" in the heart of Orlando. Hours before the shooting, the club urged partygoers to attend its "Latin flavor" event Saturday night.
Jovial, well-dressed crowds heeded the call in an event that turned into a nightmare.
Anthony Torres, who was at the club, said he and his friends were leaving the club when the shots were fired just after last call, as the club was nearing closing time.
"The shots did not sound like a regular handgun. It sounded like it was shooting repetitively and it wouldn't stop," said Torres, who added that he saw at least 10 people who had been shot.
Tearful relatives waited outside the club as lights from police cars flashed in the background. Some implored the media to help them get word on their loved ones.
The hashtags #PrayForPulse and #PrayForOrlando trended on Twitter and Instagram.
'Just a lot of people screaming'
The sound of gunshots echoed beyond the club.
Jose Torres was clocking in to work at a Dunkin' Donuts across the street when he heard them.
"It was something that I never heard before," Torres said. "I had to run inside the store, and I saw just a lot of people screaming, crying. Just screaming and coming out running like crazy."
Torres said he ducked into the Dunkin' Donuts and called 911 as several people dashed out of the club, bleeding. Police and SWAT teams rushed to the scene.
"They don't let nobody in or out," he said. "The SWAT team is inside there."
Authorities have conducted a controlled explosion near the nightclub, according to Orlando police.
Tom Fuentes, a CNN law enforcement analyst, said controlled explosions are used to breach a door or when a subject is barricaded in a room and police want to use a stun grenade to temporarily freeze their central nervous system.
FBI agents were also assisting at the scene, agency spokeswoman Amy Pittman said. President Barack Obama was briefed on the shooting and asked the FBI and other federal officials to provide him with regular updates, a statement said.
Florida Gov. Rick Scott tweeted, "My prayers are with the victims' families & all those affected by the shooting in Orlando. We will devote every resource available to assist."
Hospitals on lockdown
Authorities urged caution as police officers hovered nearby with weapons and dogs.
A few miles away, the Orlando Regional Medical Center was placed on lockdown, the hospital said in a statement. Only essential workers are being allowed access into the building.
Relatives looking for loved ones were instructed to go to the medical center's north tower with identification. Hospital staff will escort family members with IDs to a family meeting area.
Arnold Palmer Hospital and Winnie Palmer Hospital were also placed on lockdown out of an abundance of caution, but the lockdown was lifted by mid-morning, officials said.
The mass shooting occurred only a few miles from Orlando's Plaza Live theater where 22-year-old singer Christina Grimmie, who had appeared on NBC's "The Voice," was gunned down Friday night.