Her death follows a failed assassination attempt in October as she returned from exile to Pakistan. She blamed that attempt on four groups including al Qaeda and the Taliban.
A shadowy alliance of groups could also have been at work on Thursday, said Frederic Grare, a South Asia expert at the Carnegie Endowment for International Peace.
AL QAEDA DENOUNCES
Al Qaeda's second in command, Ayman al-Zawahri, this month denounced Bhutto's return as a U.S.-orchestrated manoeuvre.
"Everything that is going on in Pakistan, from the arrangement for the return of Benazir to the declaration of the state of emergency ... to repressive measures, is a desperate American attempt to remedy the deteriorating situation in Afghanistan and Pakistan," Zawahri said in an interview with al Qaeda's media arm.
Shortly before Bhutto's return in October, Taliban commander Haji Omar pledged to attack her.
Pakistan's investigation of the killing will be a major test of Musharraf's credibility, said P.J. Crowley, a former National Security Council official.
In particular, he said, the probe must make a thorough effort to identify any elements in the government who may be complicit in the attack.
The United States offered FBI assistance in investigating Bhutto's assassination, but Pakistan has not yet made a request, FBI spokesman Stephen Kodak said.
Bhutto, in an October letter to an acquaintance read on CNN on Thursday, said she would hold Musharraf responsible if she were killed, for a failure to authorize adequate security.
Rawalpindi, where Bhutto was killed, is a garrison town where Pakistan's army has its strongest grip, said RAND Corp analyst Christine Fair. "There will be those who hold him accountable even if he and his services are innocent."
U.S. State Department spokesman Tom Casey said: "It is clear that whoever is responsible is someone who opposes peaceful, democratic development and change in Pakistan."
