Pakistan failed to properly protect former prime minister Benazir Bhutto or investigate her assassination and "severely hampered" a United Nations inquiry, UN investigators said.

Bhutto was killed in a gun and suicide bomb attack after an election rally in the city of Rawalpindi on Dec 27, 2007, weeks after she returned to Pakistan from eight years in self-imposed exile.

"While she died when a 15-and-a-half-year-old suicide bomber detonated his explosives near her vehicle, no one believes that this boy acted alone," the 65-page report by a UN commission of inquiry said.

"The commission was mystified by the efforts of certain high-ranking Pakistani government authorities to obstruct access to military and intelligence sources."    

UN investigators believe the failure to effectively examine Bhutto's death was "deliberate," the report said, adding their inquiry was "severely hampered."    

"Ms Bhutto's assassination could have been prevented if adequate security measures had been taken," it concluded.   

Bhutto was mistrusted by parts of Pakistan's military and security establishment and speculation has lingered she was the victim of a plot by allies of General Pervez Musharraf, the president at the time, who did not want her to come to power.

The report, which did not name any suspected culprits but urged Pakistan to conduct a proper investigation, followed a nine-month inquiry by the three-person panel headed by Chile's UN Ambassador Heraldo Munoz.   

It was presented on Thursday to UN Secretary-General Ban Ki-moon. The release of the report was delayed for just over two weeks because of a request by President Asif Ali Zardari, Bhutto's widower, to allow the commission to hear evidence from three unidentified heads of state.

Bhutto had returned to Pakistan, a key ally to the United States in its war against Al Qaeda and the Taliban, to contest an election under a power-sharing deal with Musharraf that Washington had helped to broker.

A staunch opponent of Islamist militants, Bhutto survived a bomb attack on a rally hours after arriving home in the city of Karachi in October 2007. About 140 people were killed.   

After that bombing, Bhutto had spoken of a warning from a "friendly country" she did not identify. The UN report said Pakistan's ISI intelligence service told investigators it had received information from Saudi Arabia and United Arab Emirates about threats against Bhutto.   

The toughly worded UN report said Musharraf was aware of and tracking the many threats against Bhutto. But his government "did little more than pass on those threats to her and to provincial authorities and were not proactive in neutralizing them or ensuring that the security provided was commensurate to the threats," it said.

The report described many failures in investigating the assassination.

The Rawalpindi district police hosed down the scene and did not collect or preserve evidence, preventing a proper forensic examination. The failure to conduct an autopsy has also made it impossible to determine a precise cause of death.

The actions by police were deliberate, the report said. "These officials, in part fearing involvement by the intelligence agencies, were unsure of how vigorously they ought to pursue actions that they knew, as professionals, they should have taken," it said.

The former government that was led by allies of Musharraf blamed the late Pakistani Taliban leader and Al Qaeda ally Baitullah Mehsud for Bhutto's murder.

Mehsud was killed in a US drone strike last August. Despite the accusations against Mehsud, conspiracy theories abound in Pakistan over who was behind the assassination.   

The UN chief set up the panel in July 2009 at the request of Pakistan's coalition government, led by Bhutto's Pakistan People's Party. Its original six-month mandate was extended due to the enormity of the task.

Any criminal investigation will be up to Pakistani authorities but Munoz has said the commission's findings could complement government efforts. - Reuters