A man signs a condolence book Friday for slain Pakistani opposition leader Benazir Bhutto during a gathering to mourn her assassination in Surrey, British Columbia. Bhutto was killed Thursday in a suicide attack.

ISLAMABAD, Pakistan — Pakistan's government asserted Friday that al-Qaida was behind the assassination of Benazir Bhutto, and offered the transcript of a conversation as proof. Hundreds of thousands mobbed her funeral as the army tried to quell rioting across the nation that left 27 dead.

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.

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.