The bomb blasts targeting former premier Benazir Bhutto resembled attacks by Al-Qaeda and their allied Pakistani militants and were the work of two suicide bombers, the governor of Sindh province has said.
Governor Ishrat ul Ebad Khan said investigators have found the heads of two men that were not claimed by relatives and "almost certainly belong to the bombers".
Police had earlier said that a grenade caused the first, smaller explosion on Thursday night and that a lone suicide bomber triggered the second and bigger blast near the armoured truck in which Bhutto was travelling after her triumphant return to the port city of Karachi after eight years in self-exile.
But Khan told The New York Times that police had found no traces of a grenade and had now pieced together the head of a second suspected suicide bomber.
"Certainly these are extremists," he said. "They are the people who want to sabotage the political process. In their perspective, it would be a lethal combination for all moderate democratic forces to come together so they wanted to sabotage, disrupt and derail this process."
The bombers have not been identified but an investigator who was briefing the governor on Monday evening said the men were "100 per cent" Pakistani, the newspaper reported.
The attack was similar in style to previous suicide bombings in Karachi and elsewhere, the investigator said. The bombers used C4 plastic explosive, the same type used in the bombing of a US consulate vehicle in Karachi in March 2006, he said.
Police estimated that the first bomber was carrying 17 to 22 pounds of explosives and the second 33 pounds, the investigator said.
The Governor said the death toll had risen to 140, and included a couple with their one-year-old child. More than 500 people were wounded, he said.
Khan said that despite being aware of serious threats to her life, Bhutto had not taken necessary precautions. The governments of Pakistan and the United Arab Emirates had warned her that suicide bombers were intending to attack her cavalcade.
Security officials had tried to persuade her to take the standard precautions linked to all official and political visits -- such as not revealing the route of her procession, the vehicle she was travelling in or the time of her arrival, and moving as fast as possible from one place to another -- but Khan told the newspaper that Bhutto had not accepted their suggestions.
"They wanted a rally, they wanted to announce the time and the vehicle for political reasons," he said. "They had decided to take 18 hours to travel to the mausoleum. The home department constantly asked they reduce the time, but they did not accept this."
Khan also denied Bhutto's allegation that street lights were turned off at the time of the blasts, saying that video footage showed they were on.
Khan, who has overseen investigations into seven suicide bombings in Karachi during his five years as governor, said Bhutto had made statements before her return to Pakistan that would have caused extremists to make her a target.
She had hailed the Pakistan government's military action against militants in the Lal Masjid in Islamabad in July, and had said she would allow US to conduct operations in Pakistan.
"It was quite serious if you look at the forces of the extremists and terrorists," Khan said. "It was quite threatening to them."