State employees actively furthered and facilitated the loyalist murder of Belfast solicitor Pat Finucane but there was no over-arching conspiracy, a Government-commissioned report has found.
Sir Desmond de Silva's review of the 1989 murder of the Catholic father of three found that collusion by the state went beyond a failure to prevent the crime.
Sir Desmond examined the role of two British agents in the murder and found that another man involved was later also recruited as an agent even though he was suspected in the UDA murder of Mr Finucane.
While the QC accused successive UK Governments of a "wilful and abject failure" to implement an appropriate legal framework for running agents within paramilitary groups, he said no minister was aware of the plot to kill the solicitor.
Prime Minister David Cameron told the House of Commons the murder was "an appalling crime" and said the degree of collusion exposed was "unacceptable".
And he said, in a message to the family: "I am deeply sorry."
Mr Cameron told MPs the de Silva report highlights "shocking" levels of state collusion.
The Army and Special Branch had advance notice of a series of planned UDA assassinations, but nothing was done, he said.
Mr Cameron said De Silva found that employees of the state and state agents played "key roles" in the Finucane murder, adding: "It cannot be argued that these were rogue agents."
The review found that the Army must take a degree of responsibility for targeting activities carried out by the UDA's Brian Nelson, said Mr Cameron.
There was a "relentless" effort to defeat the ends of justice after the killing and Army officials provided the MoD with highly misleading and inaccurate information, Mr Cameron said.
But the review found no evidence that any Government was informed in advance of Mr Finucane's murder or knew about the subsequent cover-up, he said.
Mr Cameron said the Finucane family suffered "the most grievous wrongs" and he respected their view that the de Silva review was not the right response.
But he said he disagreed with them, and said a public inquiry might not have uncovered so much information about the killing.
In his report Sir Desmond said he was in no doubt that state employees were involved in collusion with terrorist killers.
He said: "The real importance in my view is that a series of positive actions by employees of the State actively furthered and facilitated his murder and that, in the aftermath of the murder, there was a relentless attempt to defeat the ends of justice.
"My review of the evidence relating to Patrick Finucane's case has left me in no doubt that agents of the State were involved in carrying out serious violations of human rights up to and including murder.
"However, despite the different strands of involvement by elements of the State, I am satisfied that they were not linked to an over-arching State conspiracy to murder Patrick Finucane."