Benazir Bhutto was assassinated leaving a party rally in December 2007
A United Nations inquiry into the assassination of former Pakistani PM Benazir Bhutto has formally began.
It is headed by Chile's ambassador to the UN, Heraldo Munoz, and includes a former Indonesian attorney general and a former senior Irish police officer.
The inquiry will last six months and investigate the "facts and circumstances" of Ms Bhutto's death.
She was killed in December 2007 as she left a rally of her Pakistan People's Party (PPP) supporters in Rawalpindi.
The three-member inquiry team will arrive in Pakistan later this month and submit its report to the UN Secretary General in six months, reports say.
Apart from Mr Munoz, the other members of the probe team are Marzuki Darusman, the former Indonesian attorney-general, and Peter Fitzgerald, who headed an early inquiry into the assassination of former Lebanese prime minister Rafik Hariri.
Pakistan's Interior Minister, Rehman Malik, told the BBC his government thought the UN investigation was necessary to find out who was behind the attack.
"We want to know who was behind this, who had conspired it, who has financed it. And we think this was a big international conspiracy," he said.
"Obviously, there might be some actors within Pakistan or within the region, but we want really to expose the whole conspiracy, because we think that this was a kind of a beginning of an attempt to Balkanise Pakistan."
These are challenging times in Pakistan to carry out such an investigation, says the BBC's World Affairs correspondent Mike Wooldridge in Islamabad.
That is not least because the Pakistan Taliban leader Baitullah Mehsud, accused by the last government here of being behind the assassination, is the target of a two-month military offensive and his militant network has hit back with retaliatory suicide attacks.
The Taliban commander has denied having anything to do with Ms Bhutto's killing.
Her assassination left questions unresolved for many people here, but especially her own party, which is now in government.
British detectives conclude a lone attacker fired the shots
After she had narrowly escaped a double suicide bombing on the day of her return to Pakistan from self-imposed exile in October 2007, she accused what she called "enemies" and "rogue elements" in the government led by President Pervez Musharraf and in the intelligence agencies of plotting to kill her.
The UN inquiry cannot itself launch criminal proceedings, but can apparently apportion blame if it chooses to do so.
Officials say the inquiry will work "fairly discreetly".
Former Pakistani President Pervez Musharraf had blamed an al-Qaeda-linked militant for the attack and refused to seek a UN investigation.
He invited police from London's Scotland Yard to assist in the inquiry into her death.
In their report, the British detectives said they believed she died due to a severe head injury sustained as a consequence of a bomb blast.
The Pakistani investigation into her death concluded that a lone attacker fired shots at Ms Bhutto before detonating explosives, but said that bullets were not the cause of death.
Ms Bhutto's Pakistan People's Party (PPP) rejected both these versions, claiming adequate security had not been provided for Ms Bhutto, and called for a wider inquiry by the UN to establish the identity and motives of the assassins.
A PPP-led coalition defeated Mr Musharraf's allies in general elections last year.
Ms Bhutto, twice prime minister of Pakistan, lived in self-imposed exile after Mr Musharraf assumed power in 1999.
Mr Musharraf had refused to seek an UN probe
In October 2007, she returned to Pakistan to campaign for the PPP in parliamentary and provincial elections - the first to be held since President Musharraf resigned as head of the army and became a civilian leader.
Shortly after her return, she survived bomb attacks on her convoy in Karachi that killed more than 100 people.
But Ms Bhutto continued to campaign and was assassinated on 27 December at a PPP rally in Rawalpindi.
She was standing upright in her armoured vehicle, with her head exposed above the open roof escape hatch, waving to the crowd when an attacker opened fire.
Seconds later, a bomb was set off at the scene which left some 20 other people dead.