Orlando gunman was investigated for extremist ties

The gunman behind the deadly attack on a gay nightclub in Orlando, Florida on Sunday had previously been investigated for his ties to an American suicide bomber, the FBI said Sunday.

Special Agent Ronald Hopper said 29-year-old Omar Mateen, the US citizen born to Afghan parents, had been cleared by the probe, but was believed to have pledged allegiance to the Islamic State before the massacre, which left 50 dead.

"The FBI first became aware of Mateen in 2013 when he made inflammatory comments to co-workers alleging possible terrorist ties," Hooper told reporters.

"The FBI thoroughly investigated the matter including interviews with witnesses, physical surveillance and records checks.

"In the course of the investigation Mateen was interviewed twice. Ultimately we were unable to verify the substance of his comments and the investigation was closed."

Later, Mateen was interviewed by agents investigating his contacts with Moner Mohammad Abusalha, a fellow Floridian and the first US citizen to carry out a suicide bombing in Syria.

Abusalha had travelled to Syria and joined al-Qaeda's affiliate, al-Nusra Front, and drove a truck packed with explosives into a government outpost in May 2014.

"We determined the contact was minimal and did not constitute a substantive relationship or a threat at that time," Hopper said.

Security officials familiar with the investigation told Reuters that there was no immediate connection between the shooter and IS.

Asked how Mateen had been able to hold a Florida gun license and to legally purchase two guns shortly before the attack, Hopper said simply that the investigation into his supposed radical ties had been "inconclusive."

"It has been reported that Mateen made calls to 911 this morning in which he stated his allegiance to the Islamic State," he said.

"We're looking into any and all connections both domestic and international."

The Islamic State group on Sunday evening claimed responsibility for the attack, which is the deadliest mass shooting in US history.

A statement by the militant group's news agency said the operation had been carried out by an "Islamic State fighter".

However, security officials familiar with the investigation told Reuters that there was no immediate connection between the shooter and IS.

Mateen's ex-wife told the Washington Post that he was abusive and mentally unstable.

"He was not a stable person," she said. "He beat me. He would just come home and start beating me up because the laundry wasn't finished or something like that."

The couple married in 2009 but divorced in 2011. She said Mateen worked as a guard at a facility for juvenile delinquents.

Officers discovered a five-gallon bucket with chemicals capable of forming an improvised explosive device. - Santa Monica Police

In a separate incident on Sunday, a man was arrested in California with an arsenal of weapons and bomb making material he had intended to use against a Gay Pride parade in Los Angeles.

James Howell, 20, was detained in Santa Monica with multiple weapons, ammunition and bomb-making materials in his car, which had Indiana license plates, police said.

Santa Monica police chief Jacqueline Seabrooks said on Twitter that Howell told officers about "wanting to harm (the) Gay Pride event," in reference to the annual L.A. Pride parade that took place in Hollywood on Sunday amid beefed up security.

Santa Monica police said in a statement that Howell was arrested after a resident reported a prowler knocking on their door and window at around 5 am.

When officers responded, they found Howell sitting in a car with three assault rifles, high-capacity magazines and ammunition, the statement said.

"Additionally, officers discovered a five-gallon bucket with chemicals capable of forming an improvised explosive device," it added.

Seabrooks said there was no known connection between Howell and the carnage in Orlando, Florida.

Authorities said they had considered calling off the parade but then decided to go forward while beefing up security, including placing undercover police in the crowd.

The march went ahead without incident.

Agencies contributed to this report.