Omar Mateen, 29, killed 50 people and injured 53 at a gay club in Orlando, Florida, on sunday morning.

According to CNN, the man responsible for the worst massacre shooting in U.S. History; Omar Mateen, worked as a security officer since 2007 at one of the world’s largest security companies, which said it was deeply shocked by the news on Sunday. 

Further investigations are being conducted by Law enforcement authorities and the FBI, G4S said.

G4S is a U.K-based firm with roughly 600,000 employees and operations in more than 100 countries. It provides security guards, technology and other services.

The company Omar worked with learned back in 2013 that he had been questioned by the FBI; but that the inquiries were subsequently closed. Florida state records alao showed that he was a licensed security officer.

Mateen, who claimed allegiance to ISIS in a phone call to authorities around the time he carried out the attack; was shot and killed.

                                                        – CNN.