Officers blasted a hole in the wall of the Pulse club in Orlando, Florida, in a desperate effort to save revellers as Omar Mateen held hostages in a toilet.
FBI director James Comey said Mateen had come to its attention twice before the shooting, and had been investigated for 10 months from May 2013 because he had made “inflammatory and contradictory” statements about ties to terrorist groups.
Mr Comey said that, at the time, Mateen claimed family connections to al Qaida and said he was a member of Hezbollah, “a Shia terrorist organisation that is a bitter enemy of the so-called Islamic State”.
Mateen admitted making the statements reported by his colleagues, but said he did it in anger because he thought were discriminating against him.
Agents closed the first inquiry in early 2014, but Mateen’s name came up in a separate FBI probe in July that year, after he was linked to a Syria suicide bomber.
John Mina, chief of police at the City of Orlando Police Department, said Mateen barricaded himself in a toilet with around four or five hostages after shooting some of his initial victims and called the police, speaking in a “cool and calm” voice with crisis negotiators. He also confirmed that when Mateen was on the phone to officers “there was allegiance to the Islamic State”.
Authorities confirmed 49 people were killed in the shooting in the early hours of Sunday, with 53 injured. Mateen was the 50th person to die. He was killed by Swat officers when he opened fire at police after crawling out of the hole made to rescue the nightclubbers.
US president Barack Obama called the massacre — the worst mass shooting in US history — an “act of terror”.
More than 300 people were in the club when gunfire was first heard just after 2am local time. He repeatedly fired, before taking dozens of people hostage, leading to a three-hour stand-off.
He fired on officers with a handgun and a “long gun” , before being killed.