Omar Mateen: Orlando gunman was already known to the FBI

The FBI described the killer as a ‘known quantity’

The FBI investigated Mateen in 2013 and 2014

The gunman who carried out the most deadly mass shooting in US history was a “known quantity” to the Federal Bureau of Investigation.

Omar Mateen of Port Saint Lucie, Florida, was on the FBI’s radar on two separate occasions before he was identified as the man behind the deaths of at least 50 people in the gay nightclub Pulse in Orlando.

He was a “person of interest” in 2013 and in 2014.

Speaking to The Washington Post, Mateen's ex-wife has described him as 'not a stable person' (Myspace)

The FBI opened an investigation into the 29-year-old but closed the case when it failed to produce any information that warranted a closer look, according to Ronald Hopper, the FBI assistant agent in charge.

Mr Hopper also confirmed to reporters that Mateen called 911 and talked about his allegiance to the Islamic State before the attack.

A source also told The Daily Beast that Mateen was a "known quantity" to the FBI.

He was killed after he shot more than 100 people in the nightclub on Saturday evening with an assault rifle and a handgun. He purchased the weapons legally just a few days before the attack.

His ex-wife told the Washington Post that he had repeatedly beaten her during their two-year marriage until July 2011 and he was not a “stable” person.

His father, Siddique Mateen, told NBC news that his son was angry when he saw pictures of two men kissing.

“We are in shock like the whole country,” he said.

Mateen’s former employer, private security firm G4S, where he worked from 2007, said it is co-operating fully with law enforcement.