FLORIDA (REUTERS) - In 2013, Omar Mateen had been investigated by the FBI for suspected ties to Islamist militants.

But officials say the US-born Mateen was dropped from their watch list, meaning he was able to buy firearms without the FBI being notified.

With a handgun and AR-15 semi-automatic rifle, Mateen stormed a packed gay nightclub in Orlando on Sunday (June 12), fatally shooting 49 people before police killed him, an act his Afghan-born father says is completely unforgivable.

"If even though he's my son I have to admit this is a terrorist act. I mean it is a terrorising the whole people. And I don't forgive him" said Seddique Mateen.

Mateen had worked as a security guard at this upscale gated community in Florida and his ex-wife says he aspired to be a police officer.

She also described him as "bipolar".

"He was mentally unstable and mentally ill, that's the only explanation I can give. And he was mentally disturbed," said Sitora Yusufly.

She says he beat her and would "express hatred toward everything".

Some Fort Pierce neighbours say he simply avoided talking to them.

"With him, it was just awkward stares, kept to himself" said Jason Beers, 31, a construction worker who lives upstairs from Omar Mateen.

And now, FBI Director James Comey says they are trying to uncover details of his life and decipher his motives.

"The reason for that is there are strong indications of radicalisation by this killer and of potential inspiration by foreign terrorist organisations."

Mateen has now become the perpetrator of America's worst mass shooting, during which he called 911 to pledge allegiance to Islamic State.