CALGARY -- The security company that employed both the Florida nightclub gunman and an armoured car guard who killed three co-workers in Edmonton in 2012 has defended its hiring practices, but says it cannot guarantee employees won't commit violent crimes.

Communications director Katie McLeod of the Canadian arm of U.K.-based G4S says the global security company does as much as it can to investigate prospective employees before hiring them, using both government and its own checks to screen them.

"In Canada, you must have your provincial security licence before you are even considered for employment and that follows all sorts of processes with the different provincial authorities," McLeod said Monday.

"It varies from province to province," she added.

Omar Mateen, a G4S employee in Orlando, Fla., has been identified as the gunman who killed 49 people and wounded more than 50 others in an attack early Sunday on a gay nightclub in the city.

Shares in G4S fell Monday after it emerged that Mateen worked for the firm. The company said Mateen was subject to a detailed screening and checks by U.S. law enforcement, which reported no findings to G4S.

The FBI has said he was interviewed twice in 2013 after making inflammatory comments to co-workers and, in 2014, he was found to have had ties to an American suicide bomber.

In 2012 in Edmonton, an armed G4S guard killed three co-workers and wounded a fourth while they were servicing a University of Alberta campus ATM.

Travis Baumgartner pleaded guilty to one count of first-degree murder, two counts of second-degree murder and one count of attempted murder in the Edmonton slayings and was sentenced to life in prison with no parole eligibility for 40 years.

He fled the shootings with more than $300,000 before being apprehended in British Columbia at the Canada-U.S. border two days later.

McLeod said G4S has since sold its armoured car division, which required staff to be armed, to Montreal-based rival GardaWorld.

Spokeswoman Melanie Otero said in an email Monday that GardaWorld conducts criminal background and global homeland security checks on all North American personnel and also verifies their previous three years of employment before hiring.

According to B.C.'s security licensing program website, prospective security licence applicants in British Columbia must provide digital fingerprints, be sufficiently fluent in English for their job and pass a criminal record check. It says licence applicants and licence holders must report any existing or new mental health condition for which he or she is receiving treatment.

McLeod said G4S employs a third party to undertake a five-year employment history verification on all employees and contractors before hiring.

She added that while American G4S employees carry guns at times while working, that's not true of its 9,000 Canadian employees.

"Our guards are not armed," she said.

G4S 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.

With files from The Associated Press