Shares in London-listed security firm G4S tumbled by as much as 8% on Monday amid revelations that the Florida nightclub shooter worked for the security firm.
It confirmed that Omar Mateen, 29, who gunned down 50 people at the Pulse nightclub in Orlando, had been employed by the business since 2007.
The world's biggest security company said he worked as a staff member at a gated retirement community in South Florida.
It added Mateen was put through screening and background checks when he was recruited in 2007, which "revealed nothing of concern".
G4S said Mateen's screening was repeated in 2013 and this threw up no new findings.
The firm said it was cooperating fully with the FBI and was aware that Mateen had been questioned by the law enforcement agency in 2013, and that its enquiries had been closed.
But the security firm added it had not been made aware of any alleged connections between Mateen and terrorist activities, or of any further FBI investigations.
Mateen was known to the FBI after making inflammatory comments to co-workers about extremism in 2013. He was questioned twice by the bureau and investigated again in 2014 over potential links to an American suicide bomber, but the probe was dropped after he was deemed to be low-risk.
Mateen was not under surveillance and the FBI said he had legally purchased two firearms within the last week.
A G4S statement said: "We are deeply shocked by this tragic event. We can confirm that Omar Mateen had been employed by G4S since September 10th, 2007. Mateen was off-duty at the time of the incident. He was employed at a gated retirement community in South Florida.
"Mateen underwent company screening and background checks when he was recruited in 2007 and the check revealed nothing of concern. His screening was repeated in 2013 with no findings.
"We are cooperating fully with all law enforcement authorities, including the FBI, as they conduct their investigations.
"In 2013, we learned that Mateen had been questioned by the FBI but that the enquiries 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."