Pakistani President Pervez Musharraf said Thursday there were problems with Pakistan's investigation into Benazir Bhutto's killing, and conceded that uncertainty remained over the exact cause of the former prime minister's death.

He denied accusations the military or intelligence services were involved in the attack, and implied it was the opposition leader's decision to greet supporters through the sunroof of her armored vehicle, and not a security lapse, that was responsible for her killing.

Musharraf spoke as demands for an international investigation into the bombing and suicide attack that killed Bhutto intensified. Many Bhutto supporters expressed doubt about the government's conclusion that she had been killed when a bomb blast slammed her head against her vehicle.

In an effort to blunt the criticism, he invited Scotland Yard investigators to aid Pakistan's probe and conceded that investigators may have erred in giving a precise cause of death just a day after her Dec. 27 killing.

"One should not give a statement that's 100 percent final. That's the flaw that we suffer from," he said, noting that more evidence was emerging into the attack. "We needed more experience, maybe more forensic and technical experience that our people don't have. Therefore, I thought Scotland Yard may be more helpful."

He also admitted there were shortcomings in Pakistan's handling of the case, including the hosing down of the bomb site hours after the attack, widely seen as undermining a detailed forensic examination. But he dismissed any suggestion there was a plan to conceal evidence.

"I'm not fully satisfied. I will accept that: cleaning the area. Why did they do that? If you are meaning they did that by design I would say no. It's just inefficiency, people thinking things have to be cleared, traffic has to go through," he said.

A senior police investigator, who spoke on condition of anonymity because of the sensitivity of the investigation, said that police had already secured key evidence from the scene, including the head of the suspected bomber, body parts, two pistols, and mobile phones.

Scotland Yard investigators, with their superior forensic techniques, could help determine whether either pistol was fired in the attack, he said.

Bhutto's death plunged an already volatile Pakistan deeper into crisis and stoked fears of a political meltdown as the nation struggled to contain an explosion of populist anger and Islamic militant violence.

Rioting in the wake of her death caused an estimated 80 billion rupees (US$1.3 billion, euro880 million) in damage in her home province of Sindh, said provincial home minister Akhtar Zamin.

Citing the violence, the government decided to put off crucial parliamentary polls for six weeks until Feb. 18.

The opposition urged the U.S.-allied Musharraf, a former army chief who seized power in a 1999 coup, to resign.

"Free and fair polls are impossible under his leadership," said Javed Hashmi, a senior member of former Prime Minister Nawaz Sharif's party. "Such a thing is unthinkable if he is there."

In a report on Bhutto's killing, the Brussels-based International Crisis Group research institute called on the United States to recognize that Musharraf was "a serious liability, seen as complicit" in Bhutto's death.

"It is time to recognize that democracy, not an artificially propped-up, defrocked, widely despised general has the best chance to provide stability," the group's Asia director, Robert Templer, said in a statement. "Unless Musharraf steps down, tensions will worsen and the international community could face the nightmare of a nuclear-armed, Muslim country descending into civil war."

Bhutto supporters have insisted that a U.N. probe would be the only way to reveal the truth about her slaying, and rejected a Pakistani investigation, even with British assistance.

"The regime has lost all credibility. Neither a domestic inquiry nor vague foreign involvement ... would lay to rest the lingering doubts and suspicions," said Farhatullah Babar, a spokesman for Bhutto's Pakistan Peoples Party.

Scotland Yard said it was sending a small team of officers from the Metropolitan Police's Counterterrorism Command. British Foreign Secretary David Miliband said the team would leave Britain by the end of the week.

The White House said it supported Scotland Yard's involvement, and that a U.N. investigation was not necessary now.

"Scotland Yard being in the lead in this investigation is appropriate and necessary and I don't see — we don't see a need for an investigation beyond that at this time," said U.S. presidential spokeswoman Dana Perino.

Musharraf denied a security lapse led to Bhutto's death, saying she had been allowed to choose the police superintendent in charge of her security, had four mobile units and 30 officers with her, and more than 1,000 police were deployed at the Rawalpindi rally where she was slain.

Musharraf said party officials should have stopped supporters from swarming her vehicle just before the attack, since any police action would have involved a baton charge or tear gas. He also implied her decision to greet the supporters through the sunroof contributed to her death, adding that those who remained inside were unharmed.

"Who is to be blamed for her coming out of her vehicle?" he asked.

Musharraf also denied reports al-Qaida was getting stronger in Pakistan, but said the country faced an increasing threat from Taliban militants.

He blamed Islamic militant leaders Baitullah Mehsud and Maulana Fazlullah for 19 suicide attacks in the past three months. Over the same period, militant-related violence killed 400 people and wounded 900 others, he said.

A day after Bhutto's killing, government officials accused Mehsud of orchestrating the attack. A Mehsud spokesman denied responsibility. Musharraf did not directly blame Mehsud in his news conference.

Also Thursday, a bomb attached to a bicycle went off at a bus station in southern Pakistan, wounding five people, said Faizullah Korejo, an area police chief. No one claimed responsibility for the attack.