Greg Toppo

USATODAY

The 911 call came early Sunday, as the shooting began.

The caller was a 29-year-old man, pledging his allegiance to the Islamic State terrorist group. He praised Boston Marathon bombers Dzhokhar and Tamerlan Tsarnaev, and, armed with an AR-15-style assault rifle, a handgun and what Orlando Police described as “an unidentified device,” he engaged police in a gun battle outside the Pulse, one of the city’s most popular gay nightclubs.

Across Orlando for nearly two weeks, revelers had been celebrating Gay Pride month — citywide, a week-long “Gay Days” festival had ended days earlier. One of the largest gay pride events in the world, it advertises itself as “America’s favorite LGBT vacation experience,” promising advertisers and sponsors access to more than 180,000 gay and lesbian travelers who converge on Orlando from all 50 states, Europe, South America and “dozens of other countries around the world.”

Saturday night was Latin Night at the Pulse, and though closing time was fast approaching, about 320 people were still inside dancing and drinking early Sunday.

One of them was Brand White, 30, of Orlando, who was with his cousin when shots rang out. “We are dancing and all of a sudden it just started like a rolling thunder, loud and everything went black,” he said.

White was shot in the shoulder. Hours later, he couldn’t recall leaving the club or how he got to the hospital. His cousin was unaccounted for.

Brandon Wolf and one of his friends were in the bathroom when they began hearing gunshots, perhaps 20 of them, getting closer.

"All I heard was gunfire after gunfire," Wolf told the Orlando Sentinel. "Eventually, I thought you were supposed to run out of ammunition. But it just kept going and going." They ran.

Orlando shooting at Pulse nightclub

Brett Rigas said he and his partner were dancing when they heard the shots. Rigas, shot in the arm, hid behind a bar. Five minutes later, authorities came in and told everyone to put their hands up and run out. As he left, Rigas said, he saw bodies.

The shooting soon became a hostage situation, police said, with a SWAT team summoned as police got updates from patrons trapped in the club. A few escaped through the back of the club, while others were trapped inside. Police from multiple departments — including a bomb squad — arrived at the scene and spent hours monitoring the situation.

Finally, before 5 a.m., police conducted a controlled detonation and used an armored vehicle to break through a wall at the nightclub. Eleven Orlando Police officers exchanged gunfire with the suspect, who was shot and killed. They rescued at least 30 hostages.

Orlando Police Chief John Mina initially estimated the death toll at more than 20, but after police were able to get access to the scene, Mina said 50 were dead and 53 hospitalized, by far the largest mass shooting in U.S. history.

Brevard County (Fla.) Sheriff Wayne Ivey, who took in the scene first-hand early Sunday, called it “one of the most horrific things I’ve ever seen.”

A federal law enforcement official told USA TODAY that the suspect had been identified as 29-year-old Omar Mir Seddique Mateen.

He’d actually been on law enforcement’s radar years earlier — FBI Special Agent Ronald Hopper said agents had investigated Mateen in 2013 and again in 2014 regarding terror threats, but that in both cases authorities found insufficient evidence to pursue charges.

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Hopper confirmed that Mateen had been interviewed by federal authorities three times in connection with two investigations during the past three years. In the most recent case, the FBI reviewed Mateen's alleged contacts in 2014 with Moner Mohammad Abu-Salha, an American suicide bomber from Florida who died in Syria the same year. Hopper said the case was closed when investigators determined that Mateen's contacts were "minimal.''

In a 2013 investigation, investigators interviewed Mateen twice about "inflammatory comments'' the gunman made to a co-worker about possible ties to international terrorism. That case also was closed when authorities were unable to "verify'' the comments.

Mateen was not under investigation at the time of the shooting, a status that allowed for his purchase of a handgun and an AR-15 rifle which were used in the assault.

New York-born, Mateen lived in Fort Pierce, about 120 miles south of Orlando on Florida’s Atlantic coast, and worked for the security firm G4S. His parents lived nearby, in Port St. Lucie.

He’d attended public school in Stuart, Fla., and earned an associate’s college degree in criminal justice technology from Indian River State College in 2006. He’d been married for about two years, then divorced. He had no criminal record. His family said he had a 3-year-old son.

Mateen’s father, Seddique Mir Mateen, maintains a Facebook page that prominently cites an organization called Durand Jirga. He incorporated it in Florida in November 2010 as a religious, charitable organization, and it is still listed as active in Florida records. Durand Jirga is an apparent reference to the Durand Line, the boundary that was established between Afghanistan and Pakistan in 1893.

The elder Mateen told NBC News on Sunday that the attack "had nothing to do with religion." He said his son harbored anti-gay sentiments — he’d evidently gotten angry a few months earlier after seeing two men kiss in downtown Miami.

A former Fort Pierce Police officer who once worked security with Mateen said the 29-year-old was “unhinged and unstable.”

Daniel Gilroy said he worked an early shift shift with G4S at PGA Village, a golf resort, for several months in 2014-15. Mateen took over from him in afternoons and he said Mateen frequently made homophobic and racial comments. Gilroy said he complained to G4S several times, but said the employer did nothing.

Gilroy quit after he said Mateen began stalking him via multiple text messages — 20 or 30 a day, he said. Mateen also sent Gilroy 13 to 15 phone messages daily. "I quit because everything he said was toxic and the company wouldn't do anything," Gilroy said Sunday. "This guy was unhinged and unstable. He talked of killing people."

By contrast, Eleanora Dorsi, a resident of the club, told The (Stuart, Fla.) News that Mateen was “very polite” and “always a gentleman."

Officials with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives said Mateen had bought the guns legally within the last few days.

In the wake of the shootings, police departments across the USA said they would beef up patrols near locations frequented by the LGBT community, including a planned vigil this week in Boston for victims of the Orlando attacks and a planned gay pride event in Washington, D.C. In Los Angeles, Mayor Eric Garcetti said police in Santa Monica arrested a heavily armed person who was headed to a gay pride parade there.

Mateen's father apologized for the attacks on behalf of his family, saying: "We weren't aware of any action he is taking. We are in shock like the whole country."

Aside from his son’s anti-gay sentiments, the elder Mateen expressed bewilderment over the crime.

“I don't know why he did it. He is dead, so I can't ask him. I wish I knew."

Contributing: Kevin McCoy, Kevin Johnson, Alison Young, Steph Solis, USA TODAY; Chris Bonanno, Bob Gabordi, Florida Today; Anthony Westbury, Nicole Rodriguez, The (Stuart, Fla.) News; Elliott Jones, Indian River (Fla.) Press Journal​; and the Associated Press. Follow Greg Toppo on Twitter: @gtoppo