Benazir Bhutto's supporters disputed a Scotland Yard finding Friday that a bomb, not a bullet, killed the ex-prime minister, and her party renewed its call for a full-scale international investigation into her death.

The British investigators' report, released Friday after a 2 1/2-week investigation, concluded that Bhutto suffered a fatal head injury when the force of a blast from a homicide bomber hurled her against a lever on the roof of her armored vehicle.

British investigators ruled out that the head injury could have been caused by a bullet. Pakistan's government announced a similar conclusion shortly after Bhutto's killing, which occurred Dec. 27 at the end of a political rally in Rawalpindi.

Bhutto's party has insisted that the former prime minister was shot and suspects a government cover-up because she had accused President Pervez Musharraf's political allies of plotting to kill her.

"We disagree with the finding on the cause of the death," said Sherry Rehman, spokeswoman for the Pakistan Peoples Party, who escorted Bhutto to the hospital after the attack. "She died from a bullet injury. This was and is our position."

"This gives us all the more reason for a United Nations probe to know the perpetrators, financiers, sponsors and organizers of the this crime," she said. "We are looking for more than the hand that pulled the trigger."

U.S. State Department spokesman Tom Casey said Friday that the United States views the investigation as credible.

"We don't have any reason why we would question the validity of their assessment," he told reporters.

When asked if there should be an independent investigation, possibly led by the U.N., Casey said: "It's important for people to feel that they have a clear understanding of what happened. We aren't proposing anything particular, though, and I think it would be up to the Pakistanis to decide whether they felt they needed more review or investigation of this beyond what Scotland Yard and Pakistani authorities have already done."

The bomb site was hosed down within hours of the attack, removing any chance of a detailed forensic examination, and no autopsy was performed before Bhutto was buried.

Television pictures appeared to show a gunman firing a pistol at Bhutto as she waved to supporters from the vehicle's escape hatch moments before the blast. British investigators confirmed shots were fired but said they did not cause her death.

The report also concluded the attack was carried out by a single assailant who fired the pistol and detonated the bomb. There was speculation that two attackers were involved.

Musharraf invited Scotland Yard to help establish the cause of death after rejecting calls for a U.N. probe. A summarized report was issued Friday by the British High Commission in Islamabad.

But the response of Bhutto's party suggested the British report alone would not calm the political storm surrounding her death as the nation prepares for the crucial Feb. 18 parliamentary elections, which were postponed for six weeks after her death.

"Far from easing the controversy surrounding Benazir Bhutto's death, this report is likely to stoke controversy," Farzana Shaikh, an analyst at the British think tank Chatham House, told Britain's news agency Press Association. "The investigation, however credible the findings may be, has taken place in a political context that is extremely complex."

British investigators said they had to rely heavily on X-rays and detailed examination of video footage of the attack. Nevertheless, the investigators said the evidence "that is available is sufficient for reliable conclusions to be drawn."

"In my opinion Mohtarma Benazir Bhutto died as a result of a severe head injury sustained as a consequence of the bomb-blast and due to head impact somewhere in the escape hatch of the vehicle," British government pathologist Dr. Nathaniel Cary said in the report.

U.S. and Pakistani officials believe the attack was masterminded by Baitullah Mehsud, a top Taliban militant commander with links to Al Qaeda. Mehsud leads a coalition of Islamic extremist groups fighting Pakistani forces in the lawless tribal area along the border with Afghanistan.

During a nationally televised press conference, senior police officer Chaudhry Abdul Majeed, who is heading Pakistan's own investigation, confirmed that police had arrested two "important" suspects in the killing based on information from a 15-year-old boy apprehended last month in northwestern Pakistan.

The boy told police he was among a five-man homicide squad charged with assassinating Bhutto.

The two suspects, identified only as Husnain Gul and Rafaqat, appeared in court Friday and were ordered held for 12 more days. Majeed said they appeared to have provided help to the bomber.

Bhutto's death sparked violent unrest across the country and prompted most candidates to scale back dramatically on public campaigning out of fear for their own safety.

Police on Friday reported a total of three people were killed and seven were wounded in a pair of clashes between Bhutto's supporters and members of the ruling Pakistan Muslim League-Q party in Sindh province, the former prime minister's political stronghold.