G4S, the massive security company that employed the man believed to have killed 49 people at a nightclub in Orlando, said it was aware that the FBI investigated the shooter in 2013
The firm said that it had conducted its own screenings of him but discovered "no adverse findings".
The firm said it ran two background checks on Omar Mateen, 29, who allegedly went on a hate-fuelled shooting spree. The first was when he was hired in 2007; the second came in 2013. A company spokesman said it was looking into whether that second screening was routine or done in response to the FBI's investigations. The company re-screens 15 per cent of its workforce every year.
Mateen worked for the UK-based company, one of the largest security firms in the world, for nearly nine years. At the time of the shooting, he was off duty, the company said, but had worked at a gated community in South Florida.
He also worked as a security officer at the St Lucie County Courthouse complex in Fort Pierce, a spokesman said. His ex-wife said he worked for a time as a guard at a facility for juvenile delinquents.
Mateen does not appear to have performed contract work for the federal Government. Deputy Attorney-General Sally Yates said it was her "understanding that he did not go through the background check to be able to work in federal facilities".
The FBI investigated Mateen over concerns about possible connections to terrorism, interviewing him three times. The probe did not lead to charges, and Mateen was able to legally purchase firearms days before the shootings.
The FBI first began investigating Mateen in May 2013, Director James Comey said. Mateen had made statements to his coworkers that "were inflammatory and contradictory," Comey said. He claimed a family connection to al-Qaeda and said he was a member of Hizbollah, a bitter enemy of Isis (Islamic State). He said he hoped law enforcement would raid his apartment and assault his wife and child, so he could martyr himself, Comey said.
1 G4S is one of the world's largest security firms, with 623,000 employees in 110 countries
2 It has worked with US agents on border patrol, provided security to the London Olympics and responded to the recent bushfires in Canada
3 The company, based south of London, works with private firms and local governments
4 G4S has a long history of serving as a US government contractor for agencies, such as the Departments of Homeland Security, State and Labour, and the Army
The FBI launched an investigation to determine whether Mateen was a terrorist, having him speak to confidential sources, recording conversations with him, "reviewing records from his communications" and searching for any other derogatory information.
Mateen said he did make those statements but said he did it in anger because his co-workers had been teasing him because he was a Muslim. The FBI closed its investigation after 10 months.
Nigel Fairbrass, a spokesman for G4S, said it was not clear what, if anything, the company did after the FBI told it that Mateen had been investigated by federal agents. "We're aware of the fact that this is a key question for the business," he said.
The firm said that he "was subject to detailed company screening" that it performed itself when Mateen was recruited in 2007 and again in 2013. Fairbrass said that the background checks it performed on Mateen involved a personality test, and that the firm checked his employment and education history, his driving record, performed a drug test and looked at his credit history. Mateen also conducted 40 hours of security training and 28 hours of firearms training.
John Kenning, G4S's regional chief executive for North America, said that in 2013, "we learned that Mateen had been questioned by the FBI but that the inquiries were subsequently closed. We were not made aware of any alleged connections between Mateen and terrorist activities, and were unaware of any further FBI investigations".
The company removed Kenning's statement from its website and instead published one that took out the references to the FBI, and did not mention that the company was aware of the agency's interest in Mateen. Instead, the statement left on the site simply said that Mateen "was subject to checks by a US law enforcement agency with no findings reported G4S".
Fairbrass said that the Kenning statement "hasn't been withdrawn so much as updated" but that the company stands "by the content in the earlier statement".
Mateen worked for the company since September 10, 2007. It was not clear whether he was required to carry a firearm in his duties for the firm.
"We are deeply shocked by this tragic event," Kenning said. "We are cooperating fully with all law enforcement authorities, including the FBI, as they conduct their investigations. Our thoughts and prayers remain with the victims of this unspeakable tragedy, and their friends and families."
While at G4S, Mateen was assigned as a security officer at the St. Lucie County Courthouse. Mateen last scanned his security badge to get into the building in May 2013, said Joseph Abreu, a spokesman for the county clerk of courts. Abreu did not know when Mateen started working for the county.
G4S contractors worked in general security and at metal-detector screening stations in the courthouse, jury room and other county buildings, Abreu said. The company made all hiring and staffing decisions, including on background checks, he said.
Bryan Beaty, a spokesman at the St Lucie County Sheriff's Office, said he believed Mateen worked at several county locations as a security guard but did not know how long the G4S contract was in place.
G4S has received about US$370 million for work in Florida since 2010, according to a review of federal contract databases. In that time frame, it received more than US$831 million nationwide, much of it from the Department of Homeland Security. Although most of its revenue comes from the United Kingdom and Ireland, about 20 per cent comes from the United States, according to its website.
G4S also held a contract reportedly worth US$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, forcing the British Government to call out 4700 troops. 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".