Northern Ireland police failed to protect a Catholic attorney at particularly high risk of assassination because of her work representing IRA and other Irish republican clients, a six-year investigation into the 12-year-old killing concluded Monday.

The fact-finding probe into the March 15, 1999, slaying of lawyer Rosemary Nelson was one of several that Britain has authorized in the past decade into Northern Ireland's most controversial killings. In all cases, police or British Army intelligence agents have been accused of aiding the killers.

Britain's secretary of state for Northern Ireland, Owen Paterson, said investigators had failed to prove any direct involvement by the territory's forces of law and order in the attack on Nelson. She had particularly attracted the police's ire by representing a reputed Irish Republican Army member, Colin Duffy, who was acquitted of killing two policemen in 1997.

But Paterson said the probe had established that police did place Nelson at unnecessary risk of assassination because they failed to advise her — as they typically did for potential targets — of ways to minimize an attack.

Paterson also told the House of Commons in London that the findings also left open the possibility that "a rogue member or members" of the police or army did assist Nelson's killers.

"If Rosemary Nelson had been given advice about her safety and offered security measures then, assuming that she had accepted such advice and security measures, the risk to her life and her vulnerability would have been reduced," Paterson said.

"I am profoundly sorry that omissions by the state rendered Rosemary Nelson more at risk and more vulnerable. It is also deeply regrettable that, despite a very thorough police investigation, no one has been charged for this terrible crime," he said.

From the start, Irish Catholic politicians and human rights activists have accused anti-Catholic extremists within the police of playing a role in her killing. Before her death, Nelson testified to United Nations and other officials that she had received about 20 death threats from officers, including those who stopped her at road checkpoints.

Nelson died after a booby-trap bomb exploded under her car, blowing off both of her legs, as she was driving to pick up her young children from school and day care. None of her three children was in the car at the time.

She was one of two prominent lawyers to be killed during the three-decade Northern Ireland conflict because she specialized in representing people hostile to the territory's predominantly Protestant police force.

The other victim, lawyer Pat Finucane, was gunned down in front of his wife and children at his Belfast dinner table in 1989. Finucane was the brother of IRA members and had successfully defended IRA suspects in a string of trials.

External investigations have already determined that Finucane's killers in the Ulster Defense Association, the major outlawed Protestant paramilitary group in Northern Ireland, were aided by both police and British Army agents seeking to influence UDA attacks at the time.

In Nelson's case, her killing was claimed by an ill-documented Protestant gang called the Red Hand Defenders — widely considered a cover name for members of the Ulster Defense Association who didn't want to admit breaking their group's cease-fire of the day.

Suspicions of security-force involvement were fueled, in part, by the relative sophistication of the attack, because the UDA demonstrated little ability to construct such bombs. Nobody was ever charged with her murder.

Monday's report — produced by a retired English judge, senior civil servant and former Welsh police chief — said the Northern Ireland police failed to prevent its officers from verbally abusing and threatening Nelson, which had the effect of "legitimizing her as a target."

Their report also cited evidence of "some leakage of intelligence" from the police to Protestant outlaws.

Eunan Magee, Nelson's brother, expressed frustration that Northern Ireland authorities appeared unwilling to bring any charges against police officers allegedly involved in threatening his sister.

"Why after so many years have the people who wronged Rosemary walked away with impunity?" he said. "This is not a closed book and hopefully charges will be brought against people who are guilty."

A fact-finding report is still pending into the Protestant mob beating of a Catholic man, Robert Hamill, in 1997. Police units are accused of standing by while Hamill was attacked and of failing to gather evidence against the attackers both at the scene and afterward.

Relatives of the other slain lawyer, Finucane, have blocked British attempts to mount a fact-finding probe into their case. They have demanded a fully independent inquiry run by non-British officials.