Former police agent William Stobie, who was accused of murdering Belfast solicitor Pat Finucane, was today shot dead, police sources said.
Loyalist paramilitary group the Red Hand Defenders have claimed responsibility for the shooting.
Stobie, an ex-Ulster Defence Association quartermaster, was gunned down at his home in Forthriver Road, north Belfast.
Police sources said he was shot at close quarters at his flat where he lived with his common-law wife.
The murder comes just weeks after his trial for his part in the killing of Mr Finucane collapsed when the prosecution's key witness refused to give evidence.
Alan McQuillan, Assistant Chief Constable of the Police Service of Northern Ireland, condemned the killing as another dreadful murder in the run-up to Christmas.
He said the victim was gunned down as he approached his car shortly after 6am.
"It's too early to speculate about what's going on here or how many gunmen were involved," he said.
"But we will be doing everything we can to investigate this and bring the people responsible to justice."
Stobie's partner is understood to have been at home when the shooting occurred.Mr McQuillan added: "She was pretty hysterical afterwards."
The murdered man's body was still at the scene as police forensic experts began examining the area.