Ken Barrett pleaded guilty two years ago to the 1989 murder of Pat Finucane, who specialized in defending Irish Republican Army suspects.

Barrett received a 22-year prison sentence for his role in the attack, in which two gunmen from the outlawed Ulster Defense Association broke into Finucane's home and shot the lawyer 14 times at point-blank range as he sat down to Sunday lunch with his wife and three children.

Barrett, 43, walked free at midday from Maghaberry Prison west of Belfast – his residence since shortly after his arrest in England in May 2003. He had been kept in isolated confinement because of death threats from his former UDA comrades.

The Good Friday peace accord of 1998 promised early paroles for convicted members of groups that observed a truce. The UDA killed more than 300 Catholics before calling a 1994 cease-fire. More than 500 members of outlawed Protestant groups and the Irish Republican Army, the major Catholic-based paramilitary group, walked free from British and Irish prisons under the peace accord.

Freedom for Barrett, who had been a paid police informer, could clear the way for a judicial probe into allegations that intelligence officers from the Northern Ireland police and British army colluded with Finucane's killers. Britain pledged such an investigation in September 2004 but it has yet to happen.

The proposed probe would allow the British government to require some evidence to be gathered behind closed doors, and information in the final report to be blacked out if it violated Britain's national security interests.

Finucane's widow and sons – who have campaigned for more than a decade for a fully public probe into possible collusion – say the British terms are too secretive and would allow the full extent of state involvement to be suppressed.

Alban Maginness, a moderate Catholic politician and Belfast lawyer, said Britain had failed “to honor its commitment to a public, independent inquiry.” He said this failing “must be challenged, exposed and overturned. The family deserve and justice demands nothing less.”

Barrett told an undercover BBC television crew in 2002 he was one of Finucane's killers and claimed that his Northern Ireland police “handlers” had urged him to make Finucane a target. The BBC documentary spurred police to launch an undercover operation against Barrett that led to his arrest and guilty plea.

A former Northern Ireland detective, Johnston Brown, claims that Barrett confessed his role in the killing in 1991, but that other officers confiscated his tape recording of the admission in order to protect Barrett.

Ken Stobie, the UDA member who admitted supplying the two handguns used to kill Finucane, was also on the police informer payroll. Stobie was killed by former UDA colleagues after being acquitted of Finucane's murder in 2001.

Other court proceedings have revealed the British army's top agent within the Belfast UDA in the late 1980s, Brian Nelson, was responsible for picking and researching UDA targets.