Security firm G4S was sent warning emails about an employee, Danny Fitzsimons, before he killed two co-workers in Baghdad in 2009.
The firm, which earned widespread criticism for failing to provide enough security staff for the 2012 Olympic Games, was sent two emails about Fitzsimons' behaviour by a "concerned member of the public and father" before he was hired, a BBC investigation found.
Fitzsimons subsequently murdered private security guard Paul McGuigan, from the Scottish Borders, and Darren Hoare, from Australia. Both were shot dead while Fitzsimmons was working on a protection contract for G4S, which was operating as the ArmorGroup in Iraq.
One email said: "I am alarmed that he will shortly be allowed to handle a weapon and be exposed to members of the public.
"I am speaking out because I feel that people should not be put at risk."
Another email read: "Having made you aware of the issues regarding the violent criminal Danny Fitzsimons, it has been noted that you have not taken my advice and still choose to employ him in a position of trust. I have told you that he remains a threat and you have done nothing."
Fitzsimons had also been diagnosed with post-traumatic stress disorder and had a criminal record. At the time of employment by G4S, he was facing charges of assault and a firearms offence. He had also lost his previous position with a security contractor for punching a client.
In the documentary, titled Britain's Private War, McGuigan's parents call for G4S to face charges over their son's death.
Corinne Boyd-Russell, his mother, said: "[Fitzsimons] fired the bullets. But the gun was put in his hand by G4S ArmorGroup. They put the gun in that man's hand. I want G4S to be charged with corporate manslaughter and be held accountable for what they did."
A spokesman for G4S said the screening process for Fitzsimons had not been completed to the normal standard.
"We confirmed publicly on September 15 2009 that, in this particular case, although there was evidence that Mr Fitzsimons falsified and apparently withheld material information during the recruitment process, his screening was not completed in line with the company's procedures.
"Our screening processes should have been better implemented in this situation but it is a matter of speculation what, if any, role this may have played in the incident.
"We are aware of the allegation over emails but following an internal IT investigation it is clear that no such emails were received by any employee before the incident. It is not for G4S to comment on the appropriateness of any criminal investigation."
Fitzsimons is currently serving a 20-year sentence for murdering McGuigan and Hoare. An inquest into McGuigan's death is due to begin in December.