Criminal investigation

Orlando nightclub shooter worked at one of the world’s largest security firms

G4S, the company that employed the man believed to have killed 50 people at a night club in Orlando is one of the world's largest security firms, with a workforce rivaling the population of Washington, D.C., that has worked with U.S. agents on border patrol, provided security to the London Olympics and responded to the recent wildfires in Canada.

During his almost nine years with the company, Omar Mateen, who allegedly went on a hate-fueled shooting spree early Sunday, worked for a time as a guard at a facility for juvenile delinquents.

In a statement Sunday, John Kenning, G4S's regional chief executive for North America, confirmed that Mateen had worked for the company since September 10, 2007.