Who is the man blamed by Pakistan's government for Bhutto's murder?
Baitullah Mehsud, 34, is the leader of a group of jihadist militants from the Afghan border area of south Waziristan.
He has launched a number of attacks on US forces across the border and has acknowledged links with the Taliban. Islamabad describes him as a senior Al Qaeda terrorist. However, he denies responsibility for the attack.
Why are the Pakistani authorities sure he is planner of Bhutto's killing?
Western diplomats say they have no reason to doubt a transcript of a conversation, said to be between Mehsud and another man, which the Pakistani authorities produced shortly after the attack and which appears to implicate him.
In the transcript, a man identified as Mehsud is heard congratulating one Maulvi Sahib, who then names two alleged killers.
Why do Bhutto's supporters suspect that Al Qaeda was not involved?
Many people inside the PPP and beyond are deeply suspicious, not only of President Pervez Musharraf, but of the country's intelligence agencies.
The speed with which the government accused Al Qaeda did little to allay fears of state involvement, and conflicting accounts of the cause of death have convinced many of a cover-up.
How is it being investigated?
Although the Pakistani authorities have already named their prime suspect, three investigations have been launched.
The first is to be headed by a judge who will be nominated by Bhutto's husband, Asif Zardari. The second is a criminal investigation, nominally co-ordinated by the Punjab provincial police, but probably dominated by the county's main intelligence agency, the Inter Services Intelligence.
The third investigation is an internal interior ministry review of the government's security precautions for Bhutto.
Why has the presence of a gunman become the cause of such bitter dispute?
The PPP says the presence of a gunman would suggest that Al Qaeda was not behind the attack, pointing to the terrorist network's frequent use of lone suicide bombers.
Others say Al Qaeda may have also dispatched a gunman after two suicide bombers operating without a gunman failed to kill Bhutto in a double attack last October in which at least 139 people died.