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.
"We are shocked and saddened by the tragic event that occurred at the Orlando nightclub," he said. "We are cooperating fully with all law enforcement authorities, including the FBI, as they conduct their investigation. Our thoughts and prayers are with all of the friends, families and people affected by this unspeakable tragedy."
G4S is based south of London in Britain and has 623,000 employees in 110 countries. It has a long history of serving as a U.S. government contractor, working with many agencies, including the departments of Homeland Security, State and Labor and the Army. While most of its revenue comes from the United Kingdom and Ireland, about 20 percent comes from the United States, according to its website.
The company has received about $370 million for work in Florida since 2010, according to a review of federal contract databases. In that same time frame, it received more than $831 million nationwide, much of it from the Department of Homeland Security.
G4S also held a contract reportedly worth $355 million to provide security for the 2012 London Olympics. But less than a month before the opening ceremony, it said was not able to provide the number of guards required. At the time, chief executive Nick Buckles reportedly said, "Clearly we regret signing it but now we have to get on and deliver it." He also agreed that it was a "humiliating shambles" for the company.
In the United States, G4S provides support to federal and local law enforcement. It has contracts with the Department of Homeland Security to provide border protection and works with Immigration and Customs Enforcement to transport illegal immigrants. It says its employees have backgrounds in the military or law enforcement or hold degrees in the criminal justice field.
G4S also does extensive work with the oil and gas and retail industries and provides security at ports and airports.
The company says on its website that it "requires applicants to go through psychological, background and criminal screening. Every G4S security officer participates in a training program that far exceeds industry standards, becoming a specialist trained to both recognize and react to unique hazards of an assigned post."
Steven Rich contributed to this report.
Editor's note: Remarks from a co-worker of Mateen were removed from this story after the allegations in those quotes could not be confirmed.