ISLAMABAD — Benazir Bhutto died from a skull fracture suffered when her head slammed against her car during a suicide attack — not from bullet wounds, the government said today.

Pakistan’s interior minister blamed Al Qaeda and the Taliban for yesterday’s assassination and said another key opposition leader, Nawaz Sharif, is also under threat of militant attack.

The government released a transcript today 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,” Mr. Mehsud said, according to the transcript.

Authorities yesterday said Bhutto died from bullet wounds fired by a young man who then blew himself up, killing 20 other people. A surgeon who treated her said today she died from the impact of shrapnel on her skull.

But later today, an Interior Ministry spokesman, Javed Iqbal Cheema, 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 the shock waves from the blast knocked her head into a lever attached to the sunroof, fracturing her skull.

At a news conference, Mr. Cheema played a videotape of the attack showing Bhutto waving, smiling and chatting with supporters from the sunroof as her car sat unmoving on the street outside a campaign rally. Three gunshots rang out, the camera appeared to fall, and the tape ended.

Bhutto was slain while campaigning for the crucial January 8 parliamentary elections in which she hoped to return as prime minister of the nuclear-armed country, a key American ally in the war on terrorism.

Upon her return from exile in October, she survived an assassination attempt. She had repeatedly complained that the government of President Musharraf did not give her adequate security. As word of her death spread, her supporters blamed Mr. Musharraf of complicity in her assassination.

Today, Interior Minister Hamid Nawaz said that the government had evidence that Al Qaeda and Taliban were behind the suicide attack. Later, Mr. Cheema blamed Mr. Mehsud, described him as an “Al Qaeda leader” and said he was also behind the October 18 bombing against Bhutto’s homecoming parade through Karachi that killed more than 140 people.

Mr. Mehsud is a commander of pro-Taliban forces in the lawless Pakistani tribal region South Waziristan, where Al Qaeda fighters are also active. His forces often attack Pakistani security forces. This fall, he was quoted in a Pakistani newspaper as saying that he would welcome Bhutto’s return from exile with suicide bombers. Mr. Mehsud later denied that in statements to local television and newspaper reporters.

Mr. Cheema said Mr. Mehsud was “behind most of the recent terrorist attacks that have taken place in Pakistan.” He said Pakistani security forces would hunt down those responsible for Bhutto’s death.

Mr. Cheema also said Mr. Sharif — also a former prime minister and now the most prominent opposition leader in Pakistan — was among several politicians under threat of militant attack. He named others as the leader of an Islamist opposition party, Fazlur Rehman; a former railways minister and ally of Mr. Musharraf, Sheikh Rashid Ahmed, and a former interior minister, Aftab Sherpao, who narrowly escaped a suicide bombing last weekend that killed 56 people.