ISLAMABAD, Pakistan (CNN) -- Former Pakistani Prime Minister Benazir Bhutto died when the force of a bomb blast slammed her head into an escape hatch on her SUV, Scotland Yard said in a report released Friday.

Benazir Bhutto greets her supporters at the rally where she was later killed. more photos »

She was not shot, the report said.

Investigators concluded that one attacker fired shots and then detonated a powerful suicide bomb as Bhutto stood through the open roof of her SUV, waving to supporters.

The force of the blast, in Rawalpindi on December 27 last year, propelled her head into part of the escape hatch and caused an injury that killed her, Scotland Yard said.

Those findings are consistent with assertions that Pakistani authorities have made since initially suggesting that she died of a gunshot wound or from an injury caused by a bomb fragment.

But a spokeswoman for Bhutto's Pakistan's People's Party said the British investigators were limited in their ability to investigate the killing fully and that the party remained convinced that Bhutto died from a gunshot wound.

The Scotland Yard report, by a team assisting Pakistani investigators examining the slaying, acknowledged "speculation that two individuals were directly involved in the attack," with one firing a gun and another detonating a bomb.

The investigators, however, concluded that only one person was directly involved in the assault that killed the leading opposition figure about two months after she returned to Pakistan after several years of self-imposed exile.

Watch Bhutto's party react to the findings »

"All the available evidence points toward the person who fired the shots and the person who detonated the explosives being one and the same person," the report says.

The attacker fired gunshots before detonating a bomb at the rear of a vehicle in which Bhutto was traveling, the report added.

"The blast caused a violent collision between her head and the escape hatch area of the vehicle, causing a severe and fatal head injury," it said.

Bhutto was killed while standing in an moving armored car after rallying supporters for parliamentary elections. The vote, originally scheduled for early January, was postponed until February 18 in the wake of her death.

Her head was above the sunroof and unprotected at the time of the attack.

The cause of her death was not clear after her death: a bomber blew himself up near Bhutto's limousine, and videotape showed a gunman apparently firing shots toward her -- but no autopsy was carried out at the family's request.

Bhutto's family and party have accused Musharraf's government of having a role in her death, and have criticized the security provided to her by the government. Musharraf has denied any involvement in her death.

Pakistani investigators examining Bhutto's slaying are being assisted by a small team from Scotland Yard.

On Friday, authorities in Pakistan said four people were in custody in connection with Bhutto's assassination, though they declined to elaborate.

Sherry Rehman, a spokeswoman for Bhutto's Pakistan People's Party, told CNN less than an hour after the report's release that she and other Bhutto supporters remained skeptical of the cited cause of death.

"We find it difficult to agree with the finding by Scotland Yard that she died from the impact of the blast," she said.

Yet she said the party wanted to look beyond the hand that pulled the trigger to focus on who organized and paid for the killing.

"This gives us all the more reason to look ... to who perpetrated the crime who financed and organized this brutal assassination," she said. "This will give us all the more impetus to ask for a larger U.N. international probe."

Pakistan's government has concluded that Bhutto's assassination was orchestrated by Baitullah Mehsud, the leader of the Pakistani Taliban who has ties to al Qaeda -- a conclusion that the CIA came to as well. E-mail to a friend

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