The CIA has added its support to the view that a Pakistani militant, Baitullah Mehsud, and al-Qaeda organised Benazir Bhutto's killing.
There are differing accounts of what caused Ms Bhutto's death
CIA Director Michael Hayden told the Washington Post that the former Pakistani prime minister was killed by fighters allied to Baitullah Mehsud.
The Pakistani government accused Mehsud of the attack shortly after Ms Bhutto's death in Rawalpindi on 27 December.
Mr Hayden did not reveal the sources for his claim.
Correspondents say that Mr Hayden's comments are the most comprehensive public assessment by US intelligence of Ms Bhutto's death.
Controversy still surrounds the circumstances of the killing.
One or more attackers shot at her and detonated a bomb as she was leaving a rally in Rawalpindi.
Baitullah Mehsud has denied involvement. The Pakistani government says it intercepted a phone conversation proving that he was behind the attack.
CIA Director Michael Hayden is now backing the Pakistani government's view.
"This was done by that network around Baitullah Mehsud. We have no reason to question that," Mr Hayden told the Washington Post.
He said the murder was "part of an organised campaign" that has included suicide bombings and other attacks on Pakistani leaders.
Mr Hayden said that the same forces were responsible for a new outbreak of violence sweeping across Pakistan which was undermining the stability of President Pervez Musharraf's government.
"You've got this nexus now that probably was always there in latency but is now active: a nexus between al-Qaeda and various extremist and separatist groups," Mr Hayden said.
"It is clear that their intention is to continue to try to do harm to the Pakistani state as it currently exists."
Forces belonging to Baitullah Mehsud have also been blamed by the government for seizing a fort from the military in the troubled region of South Waziristan on Wednesday.