G4S shares slide on news Florida gunman was employee

(Adds further details, updates shares)

LONDON, June 13 (Reuters) - G4S (Copenhagen: G4S.CO - news) shares fell as much as 8 percent on Monday after the disclosure that one of its employees was the gunman who killed 49 people at a packed gay nightclub in Florida.

Omar Mateen, 29, a Florida resident and U.S (Other OTC: UBGXF - news) . citizen, had undergone company screening as recently as 2013 with "no findings," the British company said on Sunday.

G4S stock had lost as much as 8 percent in early trading on Monday, hitting its lowest level since 2009. It (Other OTC: ITGL - news) closed almost 5 percent lower at 178.4 pence, knocking about 150 million pounds ($283 million) off the market value of the world's largest security firm.

An analyst who declined to be named said the incident could cause more damage to the G4S brand after other problems, with questions likely over the company's employee screening.

A source close to G4S said the firm carried out comprehensive checks for roles like Mateen's and that he would have been subject to at least 10 different vetting processes, including a psychological evaluation widely used in the U.S., a series of background checks, plus the submission of five different work references.

At the time of the incident, Mateen was employed at a gated retirement community in South Florida, in a role which the source noted was unarmed.

G4S said in its statement that Mateen 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 pound charge on loss-making British government contracts.

($1 = 0.7072 pounds) (Reporting by Sarah Young, additional reporting by Alistair Smout; Editing by Louise Heavens and Alexander Smith)