LONDON — Nearly a quarter of a century after one of the most brazen killings in Northern Ireland’s decades of sectarian violence, Prime Minister David Cameron of Britain said Wednesday that there had been “a shocking level of state collusion” and offered an apology to the family of the slain man.

But he stressed that there was “no overarching state conspiracy” in the episode, in which a lawyer representing Irish Republic Army activists, Patrick Finucane, was shot dead while having a Sunday dinner at his Belfast home with his wife and three children. That assertion was rejected, bitterly, by Mr. Finucane’s widow.

Speaking in the House of Commons after the publication of a 500-page, government-commissioned report by a prominent human rights lawyer into the killing, Mr. Cameron described the act as “an appalling crime.” Mr. Finucane was killed in February 1989, when two gunmen from a Protestant paramilitary group broke down a front door and fired 14 shots at him as he sat at his dinner table. He was 39....