The British security services company G4S Shares plunged heavily after it said Omar Mateen, who killed 50 people and left 53 injured at a gay nightclub in Florida city of Orlando, was an employee.

G4S shares dropped by as much as 7.5 per cent and were the biggest fallers among leading British shares. The company employs 620,000 people in more than 110 countries and the US is one of its biggest markets, the Guardian reported.

Jasper Lawler, an analyst at CMC Markets, a City spread betting firm, said it did not look good for G4S to have employed Mateen while he was being investigated by the FBI.

The world’s biggest security firm said Mateen was employed since September 2007 and he was off duty during the deadliest mass shooting in the US’ history on Sunday morning.

Afghan-origin Omar Mateen, 29, of Fort Pierce in Florida, carried an assault rifle and a pistol into the packed Pulse club, which describes itself as “the hottest gay bar”, about 2 a.m. and began shooting.

The Islamic State militant group said it was behind the attack, but the extent of its involvement is not clear.

G4S said it carried out detailed screenings on Mateen when employed and again as recently as 2013 “with no adverse findings”.

Mateen was interviewed three times by Federal Bureau of Investigation in 2013 and 2014 following reports of extremist behaviour and connections to terrorism that were ultimately judged to be insubstantial.

“He was subject to checks by a US law enforcement agency with no findings reported to the company. G4S is providing its full support to all law enforcement authorities in the US as they conduct their investigations,” the company said.

In a statement, John Kenning, G4S’s North America chief executive, said: “We are cooperating fully with all law enforcement authorities, including the FBI, as they conduct their investigation. Our thoughts and prayers are with all of the friends, families and people affected by this unspeakable tragedy.”

The company said Mateen was an armed security official and it was trying to establish whether weapons used in the attack on the Pulse gay nightclub were related to his work.

“G4S has more than 50,000 employees in the US, a large proportion of which are involved in government contracts. If the name of G4S starts getting dragged through the mud, US contracts may become harder to come by.”

The company, whose activities range from supplying bodyguards and guarding buildings to running prisons and clearing mines, has been dogged by controversy.

Two years ago it agreed to repay the British government 109 million pounds after it overcharged for the electronic tagging of offenders.

As many as three G4S security guards were acquitted of manslaughter charges in 2014 concerning the death of a man from a cardiac arrest on a flight from London.

In 2014, the Bill and Melinda Gates Foundation said it had sold its stake in G4S.