The men were seized on 29 May 2007
The two British hostages whose bodies were returned from Iraq last week had been shot, the BBC has learned.
Security guards Jason Swindlehurst, from Skelmersdale, Lancashire, and Jason Creswell, from Glasgow, died from gunshot wounds, sources said.
The men's families were informed on Sunday night and they, in turn, told the families of the other three British hostages still missing in Iraq.
The five men were captured in Baghdad in May 2007.
BBC security correspondent Frank Gardner said the revelation of the men's apparently violent deaths some time ago is a bitter blow to all the families who are still awaiting news of the three remaining hostages.
The five men were captured at Baghdad's Ministry of Finance by a group of about 40 men disguised as Iraqi policemen.
The three still thought to be in captivity are two other security guards and Peter Moore, the IT consultant whom they were protecting.
Mr Moore had been working for American management consultancy Bearingpoint, while the other men were contractors employed to guard him.
The bodies of Mr Swindlehurst and Mr Creswell were flown to RAF Lyneham in Wiltshire last Friday.