Shares in the security company G4S have fallen over 5 percent after it emerged that the shooter in the Orlando nightclub attack had worked for the firm.

The company's shares fell 9.6 pence to 177.80 on Monday after the company acknowledged that Omar Mateen worked for G4S at a residential community in south Florida. The company said Mateen was subject to a detailed screening and checks by U.S. law enforcement, which reported no findings to G4S.

G4S, a global security company, is active in some 100 countries and has 610,000 employees.

It came under fire during the 2012 London Olympics after failing to provide the number of security guards promised to protect the games. The British military had to be called in to fill the gap.