Who killed Benazir Bhutto? In a country riven by deep political differences and beset by constant violent threats from Islamist terrorists, including Al Qaeda, it is a question more easily asked than answered.

American officials are exploring whether the immediate claim of responsibility by an Al Qaeda leader in Afghanistan posted on an Italian Web site is credible. A reporter from the Web site spoke to the commander of Al Qaeda forces in Afghanistan, Mustafa Abu al-Yazid, who said, “We terminated the most precious American asset which vowed to defeat [the] mujahideen.”

The report went on to say that the decision to assassinate Bhutto was taken in October by Osama bin Laden’s deputy, Ayman al Zawahri, who was imprisoned for his part in the plot which assassinated President Anwar Sadat of Egypt in 1981.

Al Qaeda claimed responsibility for the attempt by a suicide bomber on Bhutto’s life in Karachi on October 18, when 136 were killed and at least 450 injured. After that assassination attempt she received a letter from someone claiming to be close to Mr. bin Laden, who threatened she would be slaughtered “like a goat.”

Bhutto was a longstanding outspoken critic of Al Qaeda and what she called the Islamist “forces of militancy” and blamed President Musharraf for not doing enough to purge Pakistan of Al Qaeda fighters. Apart from her virulent opposition to their campaign of violence, Islamists disliked Bhutto, a Shia Muslim, for taking a prominent role in Pakistan’s political life.

Richard Clarke, who advised President Bush on counterterrorism, told ABC News, “She openly threatened Al Qaeda and she had American support. If Al Qaeda could try to kill Musharraf twice, it could easily do this.”

Two militant leaders in the mountainous and lawless region of northwest Pakistan where Osama bin Laden is in hiding, Baitullah Mehsud and Haji Omar, made direct threats against Bhutto’s life before she returned to Pakistan from exile in Britain and Dubai in October.

Mr. Mehsud led the fight against the Pakistani army in the tribal region of South Waziristan, is closely linked to Al Qaeda and the Taliban in Afghan. Mr. Omar, considered the “amir” or chief of the Pakistani Taliban, also based in South Waziristan, is a veteran of the mujahideen resistance against the Soviet occupation of Afghanistan.

Bhutto’s closest supporters are blaming Mr. Musharraf, who opposed Bhutto’s election as prime minister in the elections planned for January 8, for not providing her with adequate security.

They also suspect that members of Mr. Musharraf’s military government and intelligence services, who would have lost influence if Bhutto was elected, were behind the murder. Bhutto repeatedly accused Pakistani intelligence officers for conspiring with Islamist terrorists to kill her.

Heading the list of those thought capable of plotting Bhutto’s demise is the Inter-Services Intelligence, the spying agency that encouraged the Islamist opposition to the Soviet occupation of Afghanistan. The ISI is the main instrument by which Pakistan’s leaders suppress internal political opposition.

After the October 18 attempt on her life, her husband, Asif Ali Zardari, accused the ISI of being intimately involved.

The nature of the attack on Bhutto is believed to suggest that Al Qaeda was not involved. Their favorite method of assassination is by suicide bomber in a vehicle. Bhutto’s killer, however, fired three shots at her as she rode in her ostensibly bullet proof and steel protected SUV before blowing himself up.