Omar Mateen, the Orlando gunman, was investigated by the FBI for ten months after he alarmed co-workers in 2013 by claiming to be a member of Hizbollah and have family ties to al-Qaeda.

James Comey, the FBI director, said the agency interviewed Mateen, sent in undercover agents and trailed him during the course of a ten month investigation.

Mateen admitted at the time that he had told colleagues he was affiliated with the terrorist groups, but said he only made the comments "in anger" because he felt persecuted for being Muslim.

He was again investigated in 2014 after being linked to a man who became a suicide bomber in Syria. The men attended the same mosque and knew each other, but the FBI determined that they had no significant relationship.

The 29-year-old American citizen whose parents had immigrated from Afghanistan, had been in regular communication with Moner Mohammed Abu Salha, a fellow Floridian who was a member of al-Qaeda-linked Jabhat al-Nusra.

In early 2014, Mateen was put under surveillance for possible ties with Abu Salha, the son of a Palestinian father and American mother who left his home state to join jihadists in Syria.

Salha, 22, was trained by Nusra in 2012 before returning to Florida in 2014, when it is believed he met with Mateen.