Investigators probing the attack on former premier Benazir Bhutto's [Images] motorcade now feel that the twin blasts were carried out by two suicide bombers, even though the Pakistan's Interior Minister maintained that only one bomber was involved.

Investigators in the port city of Karachi have concluded that the blasts were caused by "twin suicide bombings of varying intensities," sources close to a top police investigation team were quoted as saying by the Dawn newspaper on Monday.

In the course of their probe, police have "reached an opinion that the first blast was also the result of a suicide bombing, although it was of less intensity compared with the second blast," the report said.

This assumption has been strengthened by the discovery of a second severed head, "with its one side intact", on Friday.

Police, with the help of doctors, will reconstruct the face "so that it could become identifiable", a senior police officer said adding, since the provincial police chief had said that the first blast was caused by a grenade, no police officer is willing to contradict him.

However, Interior Minister Aftab Ahmad Khan Sherpao, who is supervising the probe, said authorities were certain only one bomber, who was on foot and not in a car, had carried out the attack.

He said the first blast was caused by a grenade, but officials had not found any pieces of the grenade.

"We have recovered the trigger, the striker sleeve of the suicide bomber's detonator. It has similarities with other (suicide bombing) cases that recently have taken place," Sherpao told Newsweek.

The bomber, who used a device with up to 14 kg of explosives and a large number of pellets, was presumed to be from Pakistan's tribal areas bordering Afghanistan.

"The explosion aimed at me (in April), at the Marriott Hotel in Islamabad (in January), and at a number of other places share the same similarities. Now we are trying to determine where the suicide bombers are resourced. Naturally, the source is in the tribal area or in Swat," Sherpao said.

The police chief of Sindh province had also told the media on Friday that the first blast was caused by a grenade lobbed by the attackers to distract security personnel and the second was a suicide bombing.

Sherpao refused to comment on Bhutto's allegation that government officials like Intelligence Bureau Chief Ejaz Shah could be linked to the attack, saying: "Because of the sensitivity of my post and the ongoing investigation I will not be able to comment on this. We want to do the investigation objectively and not politicize it. So I'm not accusing anyone until we get the evidence."