ISLAMABAD, Pakistan — A pair of suicide bombers were apparently behind last week's bloody attack on opposition leader Benazir Bhutto, a top intelligence official said today.

Police had initially said only one suicide bomber participated in the attack, but the official, speaking on condition of anonymity, said two severed heads found at the scene — and the fact that no families had emerged to claim either one — suggested two men were responsible.

"One of the attackers wanted to blow himself up under Benazir Bhutto's truck. When people tried to catch him, he detonated the bomb," the intelligence official said.

Although Bhutto herself escaped unhurt, at least 136 people were killed.