ORLANDO, Florida (CNN) -- A gay nightclub here was the scene early Sunday of the worst terror attack in U.S. history since 9/11.
* 50 people were killed inside the Pulse club and at least 53 people were injured, police say.
* The gunman was Omar Mateen of Ft. Pierce, Florida, a law enforcement source told CNN.
* Orlando police shot and killed Mateen.
* Mateen called 911 around the time of the attack to pledge allegiance to ISIS and mention the Boston bombers, according to a U.S. official.
"It appears he was organized and well-prepared," Orlando Police Chief John Mina said early Sunday. The shooter had an assault-type weapon, a handgun and "some type of (other) device on him."
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.
Officials, aided by the FBI, warned that a lengthy investigation was ahead given the number of victims and the scope of the violence.
"There's blood everywhere," U.S. Rep. Alan Grayson told reporters. "There's an enormous amount of evidence to be collected."
It's just shocking," said Christopher Hansen, who was inside Pulse when he heard gunshots, "just one after another after another."
"It could have lasted a whole song," he said.
Hansen was getting a drink at the bar about 2 a.m. when he "just saw bodies going down."
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.
The City of Orlando began posting the names of victims as they had been identified and family had been notified.
Edward Sotomayor Jr., 34 years old
Stanley Almodovar III, 23 years old
Luis Omar Ocasio-Capo, 20 years old
Juan Ramon Guerrero, 22 years old
Eric Ivan Ortiz-Rivera, 36 years old
Peter O. Gonzalez-Cruz, 22 years old
Luis S. Vielma, 22 years old
Kimberly Morris, 37 years old
Eddie Jamoldroy Justice, 30 years old
Darryl Roman Burt II, 29 years old
Deonka Deidra Drayton, 32 years old
Alejandro Barrios Martinez, 21 years old
Anthony Luis Laureanodisla, 25 years old
Jean Carlos Mendez Perez, 35 years old
Franky Jimmy Dejesus Velazquez, 50 years old
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.
President Barack Obama was scheduled to make a statement about the attacks at 1:30 pm ET.
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. His ex-wife Sitora Yusufiy, described him as being violent and mentally unstable.
"To be in someway affiliated with at one point in my life with some tragedy, it shook me off the ground," she said.
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.
Ricardo Negron Almodovar said he was in the club when the shooting started. He barely escaped.
"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," he posted on the club's Facebook page.
"I am safely home and hoping everyone gets home safely as well."
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.