Shares in British private security giant G4S plunged on the London stock market Monday after it emerged that Omar Mateen, the gunman behind the massacre in a Florida gay nightclub, was one of its employees.The share price was down 5.71 percent at 176.70 pence at around 1000 GMT and the company published a statement on the London Stock Exchange website reiterating details about Mateen's employment. "Mateen was subject to detailed company screening when he was recruited in 2007," the group said in a statement to the London Stock Exchange.Mateen was "re-screened in 2013 with no adverse findings. He was also subject to checks by a US law enforcement agency with no findings reported to G4S," the statement said. That followed an FBI statement on Sunday saying it had interviewed Mateen three times in 2013 and 2014. Around 23 percent of its workforce is in Britain and Ireland and 20 percent in North America. In 2013, the group had a turnover of $10.5 billion.