WASHINGTON - Al Qaeda is the chief suspect in the murder of Pakistani opposition leader Benazir Bhutto, standing to gain by preserving its remote stronghold, undermining President Pervez Musharraf and destabilizing the country, U.S. government and private analysts said.

The militant group, which has rebuilt its command structure on the Pakistan-Afghanistan border, was blamed for a previous attempt on Bhutto and it has denounced her as an instrument of U.S. policy in Pakistan.

Bush administration officials said it was too early to identify a clear suspect in Thursday's assassination.

But one U.S. official said: "There are a number of extremist groups within Pakistan that could have carried out the attack. ... Al Qaeda has got to be one of the groups at the top of this list."

Al Qaeda's Taliban ally, which has publicly threatened Bhutto, was another potential suspect, said the official, who spoke on condition of anonymity.

One private analyst said al Qaeda supporters in Pakistan's security services may have also played a role, but it was unlikely Musharraf himself was involved.

Killing Bhutto undermines Musharraf, viewed by the United States as an essential ally against terrorism, by eliminating the prospect of a power-sharing agreement between the two that could shore up his deteriorating political standing and stabilize the country, the analysts said.

That in turn reduces chances Musharraf can revive efforts to drive al Qaeda and the Taliban out of the remote Waziristan tribal areas. It also fans popular suspicions against Musharraf and sows general confusion.

"Their (al Qaeda's) motivation for doing this is entirely clear," said David Gartenstein-Ross of the Foundation for Defense of Democracies. "They have the most to gain."

Bhutto was assassinated in a suicide bombing after an election rally in the city of Rawalpindi, ahead of January 8 national elections meant to return Pakistan to a civilian-led democracy.

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."