President Pervez Musharraf's government also said Bhutto was not killed by gunshots or shrapnel as originally claimed. Instead, it said her skull was shattered by the force of a suicide bomb blast that slammed her against a lever in her car's sunroof.
View the rest of the report as well as an onsite video of the assassination after the jump.
The new explanations were part of a rapidly evolving political crisis triggered by the death of Bhutto, Musharraf's most powerful foe in the elections. The rioting by Bhutto's furious supporters raised concerns that this nuclear-armed nation, plagued by chaos and the growing threat from Islamic militants even before the killing, was in danger of spinning out of control.
Pentagon officials said Friday they have seen nothing to give them any worries about the security of Pakistan's nuclear arsenal.
While many grieving Pakistanis turned to violence, hundreds of thousands paid their last respects to the popular opposition leader as she was placed beside her father in a marble mausoleum in the Bhutto ancestral village in southern Sindh province.
"I don't know what will happen to the country now," said mourner Nazakat Soomro, 32.
The government said it would hunt down those responsible for her death in the lawless tribal areas along the Afghan border where Osama bin Laden and other al-Qaida leaders are thought to be hiding.
"They will definitely be brought to justice," Interior Ministry spokesman Javed Iqbal Cheema said.
The government released a transcript Friday of a purported conversation between militant leader Baitullah Mehsud and another militant.
"It was a spectacular job. They were very brave boys who killed her," Mehsud said, according to the transcript. The government did not release an audiotape.
Bhutto's party rejected claims that Mehsud was behind the attack, saying the militant — through emissaries — had previously told Bhutto he was not involved in the Karachi bombing.
"The story that al-Qaida or Baitullah Mehsud did it appears to us to be a planted story, an incorrect story, because they want to divert the attention," Farhatullah Babar, a spokesman for Bhutto's party, said Saturday.
Babar said Bhutto had earlier told the government of "elements" other than al-Qaida that she thought could be a threat to her, but officials never investigated.
Cheema described Mehsud as an al-Qaida leader who was also behind most other recent terror attacks in Pakistan, including the Karachi bomb blast in October against Bhutto that killed more than 140 people.
Mehsud is thought to be the commander of pro-Taliban forces in the tribal region of South Waziristan, where al-Qaida fighters are also active.
In the transcript, Mehsud gives his location as Makin, a town in South Waziristan.
This fall, he was quoted in a Pakistani newspaper as saying that he would welcome Bhutto's return from exile with suicide bombers. Mehsud later denied that in statements to local television and newspaper reporters.
There have been no reports of reliable claims of responsibility by al-Qaida.
Cheema announced the formation of two inquiries into Bhutto's death, one to be carried out by a high court judge and another by security forces. Bhutto was assassinated Thursday evening after a rally in the garrison city of Rawalpindi near Islamabad. Twenty other people also died in the attack.
On Thursday, authorities had said Bhutto died from bullet wounds fired by a young man who then blew himself up. A surgeon who treated her, however, said Friday she died from the impact of shrapnel on her skull.
But later Friday, Cheema said those two accounts were mistaken. He said all three shots missed her as she greeted supporters through the sunroof of her vehicle, which was bulletproof and bombproof.
He also denied that shrapnel caused her death, saying Bhutto was killed when she tried to duck back into the vehicle, and that the shock waves from the blast knocked her head into a lever attached to the sunroof, fracturing her skull. The government released a photograph showing blood on the lever.
Denying charges the government failed to give her adequate security protection, Cheema said it was Bhutto who made herself vulnerable and pointed out that the other passengers inside Bhutto's bombproof vehicle were fine.
"I wish she had not come out of the rooftop of her vehicle," he said.
Bhutto's death sparked deadly rioting that killed at least 27 people, according to an Interior Ministry official who spoke on condition of anonymity because he was not authorized to speak to the media.
Rioters in the southern city of Karachi torched 500 vehicles, 13 banks, seven gas stations and two police stations, police chief Azhar Farooqi said. The violence killed 13 people, including five workers in a garment factory that was set ablaze, police said. A shootout between rioters and police wounded three officers, police said.
Another six people died from suffocation in Mirpurkhas, about 200 miles northeast of Karachi, when a bank building was set on fire, said Ghulam Mohammed Mohtaram, the top civilian security official in Sindh province.
About 7,000 people in the central city of Multan ransacked seven banks and a gas station and threw stones at police, who responded with tear gas. Media reports said 200 banks were attacked nationwide.
Vandals also burned 10 railway stations and several trains across Sindh province, forcing the suspension of all train service between Karachi and the eastern Punjab province, said Mir Mohammed Khaskheli, a senior railroad official.
An Associated Press reporter saw nine cars of a train completely burned. Witnesses said all the passengers were pulled out before the train was torched.
Desperate to quell the violence, the government sent troops into the streets of Hyderabad, Karachi and other areas in Sindh. In Hyderabad, the soldiers refused to let people out of their homes, witnesses said.
The army readied 20 battalions of troops for deployment across Sindh if they were needed to stop the violence, according to a military statement.
"We will sternly deal with those who are trying to create disorder," Cheema said.
Paramilitary rangers were also given the authority to use live fire to stop rioters from damaging property in the region, said Maj. Asad Ali, the rangers' spokesman.
"We have orders to shoot on sight," he said.
Many cities were nearly deserted as businesses closed and public transportation came to a halt at the start of three days of national mourning for Bhutto.
Prime Minister Mohammedmian Soomro said the government had no immediate plans to postpone Jan. 8 parliamentary elections, despite the violence and the decision by Nawaz Sharif, another opposition leader, to boycott the poll.
"Right now the elections stand where they were," he told a news conference.
The United States, which sees Pakistan as a crucial ally in the war on terror, was counting on Musharraf to proceed with the vote in the hope it will cement steps toward restoring democracy after the six-week state of emergency he declared last month.
Keeping the election on track was the biggest immediate concern in sustaining an American policy of promoting stability, moderation and democracy in Pakistan, U.S. officials said Friday.
Bhutto's death left her populist party without a clear successor. Her husband, Asif Ali Zardari, who was freed in December 2004 after eight years in detention on graft charges, is one contender to head the party although he lacks the cachet of a blood relative from the Bhutto clan's political dynasty.
Throughout the day, hundreds of thousands of mourners arrived in Bhutto's hometown of Garhi Khuda Bakhsh in tractors, buses, cars and jeeps for her funeral cortege and burial.
Bhutto's plain wood coffin, draped in the red, green and black flag of her Pakistan People's Party, was carried in a white ambulance toward the marble mausoleum about three miles away, passing a burning passenger train on the way.
By MUNIR AHMAD, Associated Press Writer 31 minutes ago
Associated Press Writer Lolita C. Baldor in Washington contributed to this report.