Kevin Johnson, USA TODAY and Elliott Jones, Indian River (Fla.) Press Journal​

PORT ST. LUCIE, Fla. — Investigators were reviewing a range of possible terror and hate-crime links to a gunman who professed his allegiance to the Islamic State from the scene of a horrific mass shooting at a crowded Orlando nightclub early Sunday that left at least 50 dead and 53 others wounded, the FBI said.

Omar Mateen, 29, of Fort Pierce, Fla., acknowledged his support for the terror group during a 911 call to local law enforcement from the nightclub, Orlando FBI chief Ron Hopper said.

During the call, placed in the pre-dawn hours after the first round of shots were fired, Mateen also made reference to the deadly 2013 Boston Marathon bombings, said a separate federal law enforcement official who was not authorized to comment publicly.

The disclosure closely tracked an account provided earlier Sunday by California Rep. Adam Schiff, the ranking Democrat on the House Intelligence Committee. Schiff said that a Department of Homeland Security briefing indicated that Mateen had made the radical proclamation before he died in a shootout with authorities.

“Whether this attack also was ISIS-directed remains to be determined,’’ Schiff said in a statement.

Hopper also confirmed Sunday 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.'' A federal law enforcement official later said a review of the Abu-Salha case found no direct contact between Mateen and the bomber. The two attended the same mosque, the official said.

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.

In both cases, the federal law enforcement source said, Mateen agreed to be interviewed and cooperated with investigators.

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. A Bureau of Alcohol Tobacco Firearms and Explosives trace found that the firearms were purchased "legally'' in Florida within the "last few days.''

Investigators, meanwhile, were interviewing members of Mateen’s family Sunday in an attempt to learn what may have prompted the assault, two federal law enforcement officials said.

NBC News reported that the attacker’s father indicated that Mateen recently expressed anti-gay sentiments, but one of the officials said investigators were still reviewing a wide range of possible motivations.

The official also said investigators were reviewing Mateen’s recent travels and contacts to learn more about possible preparations for the attack, now the largest mass shooting in U.S. history.

According to Florida court records, Mateen was married in 2009 and divorced two years later.

Who is Omar Mateen?

Mateen married Sitora Yusufiy on April 16, 2009. The marriage license was issued in St. Lucie County, Fla., records show. A dissolution of marriage was filed in July 2011.

Yusufiy could not be immediately reached. But in an interview with The Washington Post, the ex-wife claimed she was beaten repeatedly.

A former Fort Pierce police officer who once worked with Mateen as a security guard at PGA Village in Port St. Lucie, Fla., said Mateen was "unhinged and unstable."

Daniel Gilroy said he worked the 7 a.m. to 3 p.m. shift with G4S Security at the south gate of the community for several months in 2014 and 2015. Mateen took over from him for a later shift.

Gilroy said Mateen frequently made homophobic and racial comments. Gilroy said he complained to his employer several times and quit after he said Mateen began stalking him with up to 20 or 30 texts per day. He also left Gilroy 13 to 15 phone messages a day, the former officer said.

"I quit because everything he said was toxic," Gilroy said Sunday, "and the company wouldn't do anything. This guy was unhinged and unstable. He talked of killing people."

John Kenning, a regional G4S chief executive, confirmed that Mateen had been employed there since September 2007.

"We are shocked and saddened by the tragic event that occurred at the Orlando nightclub,'' Kenning said in a written statement. "We are cooperating fully with all law enforcement authorities, including the FBI, as they conduct their investigation. Our thoughts and prayers are with all of the friends, families and people affected by this unspeakable tragedy.''

Two of Mateen's prior acquaintances described the gunman's actions as completely out of character for the person they knew.

"He would never shoot anybody or kill anybody,'' Lamont Owens said, adding that he had not seen Mateen for a "few'' years.

Another associate, Ryan Jones, described Mateen as "normal,'' though he also acknowledged not having contact with Mateen for several years.

"He was a cool, calm and collected person,'' Jones said.

Born in New York, Mateen lived in a Fort Pierce apartment complex that was teeming with law enforcement officials Sunday. He also used a mailing address at his parents' nearby Port St. Lucie, Fla., address.

Mateen received an associates of science degree in criminal justice technology in 2006 from Indian River State College, according to college spokeswoman Michelle Abaldo.

Local law enforcement records show no apparent criminal history in Florida.

Contributing: Anthony Westbury and Nicole Rodriguez in Port St. Lucie; Johnson reported from Washington.

More coverage of the Orlando shooting:

Orlando police: 'Approximately 20 dead' after club rampage

Everyone was 'dropping and screaming': Witnesses describe chaos in Orlando shooting

Brutal rampages in U.S. history

Mass shooting creates chaos at Orlando nightclub