A U.N. fact-finding commission began an inquiry on Wednesday into the assassination of former Pakistani Prime Minister Benazir Bhutto.

Bhutto was murdered in a suicide gun-and-bomb attack in the city of Rawalpindi on Dec. 27, 2007, after a rally to drum up support for a general election she had hoped to win.

Her murder threw nuclear-armed U.S. ally Pakistan into crisis and her Pakistan People's Party rode a wave of sympathy to win the election, which was delayed until February 2008.

Her widower, Asif Ali Zardari, later become president.

The three-member U.N. team is headed by Chile's U.N. Ambassador Heraldo Munoz and will take six months for its investigation.

"It's a fact-finding mission. It has started work today and it'll just inquire into the facts and circumstances of the assassination," said the U.N. spokeswoman, Ishrat Rizvi.

While it started its work on Wednesday, the team was not yet in Pakistan but would arrive this month, Rizvi said.

The team will not be empowered to launch criminal proceedings related to the assassination.

That will make it much less far-reaching than a U.N. investigation of the 2005 killing of former Lebanese Prime Minister Rafik al-Hariri, which is intended to lead to a U.N.-organised trial in The Hague.

"It's been agreed between the government and the United Nations that the duty of determining criminal responsibility of the perpetrators of the assassination remains with Pakistani authorities," said Rizvi.

Pakistan's previous government, led by Pervez Musharraf, and the U.S. Central Intelligence Agency accused al Qaeda-linked Pakistani Taliban chief Baitullah Mehsud of killing Bhutto, a staunch supporter of the U.S.-led campaign against militancy.

Mehsud denied involvement.

ENEMIES

British police also investigated how Bhutto had been killed but not who had been responsible.

Some of Bhutto's aides have expressed dissatisfaction over the previous investigations.

Bhutto had enemies apart from Islamist militants and conspiracy theories were fuelled when authorities ordered the scene of the attack hosed down shortly after it happened, washing away evidence.

About 20 people were killed when the suicide bomber struck as Bhutto was leaving a stadium waving to supporters from the roof escape hatch of her armoured vehicle.

A spokesman for President Zardari said the government had sought a U.N. inquiry to avoid allegations of partiality. He also said the assassination had international ramifications, although he did not elaborate.

"It has ramifications and it's tentacles go far beyond the national boundary," said the spokesman, Farhatullah Babar.

"We also wanted an international independent body so there will be no allegations or accusations," he said.

The government recently ordered an offensive against Mehsud, based in South Waziristan on the Afghan border, who the army says is responsible for 90 percent of terrorist attacks in Pakistan.

He carries a U.S. reward of $5 million and a Pakistani reward of 50 million rupees ($615,000).

The other two members of the U.N. "Bhutto Commission" are Indonesia's former attorney general Marzuki Darusman and Peter Fitzgerald, a retired senior officer with the Garda Siochana, Ireland's national police force.