News that a G4S employee was the gunman who killed 50 people at a packed gay nightclub in Florida wiped almost £200 million ($282.80 million) off the value of the world's largest security firm.
Omar Mateen, 29, a Florida resident and U.S. citizen, had undergone company screening as recently as 2013 with "no findings," his Britain-headquartered employer G4S said on Sunday.
When trading opened in London on Monday, shares in G4S lost 6.6 percent to 175 pence to hit their lowest level since 2009.
An analyst who declined to be named said the incident could cause more damage to G4S after a long line of issues.
"It doesn't help having their name in the press against something like that," the analyst said.
Mateen was employed at a gated retirement community in South Florida. He underwent two instances of company screening and background checks - once when he was hired in 2007, and again in 2013. At that time, the company learned that Mateen had been questioned by the FBI but that the inquiries were then closed.
G4S has a chequered recent history after it failed to provide enough guards for the London Olympics in 2012, was then involved in a tagging fiasco the following year, and earlier this year took a £65 million charge on loss-making British government contracts.