Omar Mateen, 29, killed 50 people and injured 53 others at a gay club in Orlando, Florida, early Sunday morning, according to authorities. Mateen worked at a gated retirement community in South Florida, according to G4S. The man behind the worst mass shooting in U.S. history worked since 2007 as a security officer at one of the world's largest private security companies, which said Sunday it was "deeply shocked" by the news. "We are cooperating fully with all law enforcement authorities, including the FBI, as they conduct their investigations," G4S Secure Solutions said in a statement. G4S said that Mateen started working for it in September 2007 and that company screening and background checks "revealed nothing of concern."

Source: CNN June 12, 2016 19:52 UTC