"Let me reiterate that if you think that there will be smoking guns in terms of names, our report is not that and will not be that," Heraldo Munoz, head of the three-member UN commission, told a press conference in Islamabad.
"The mandate does not include a criminal investigation," confirmed Munoz, flanked by Indonesia's Marzuki Darusman and Ireland's Peter Fitzgerald at their first public appearance in Pakistan since opening a six-month inquiry.
Bhutto, the first woman to become prime minister of a Muslim country, was killed on December 27, 2007 in a gun and suicide attack after addressing an election rally in Rawalpindi, a garrison city near the capital Islamabad.
The government called for a UN inquiry after Bhutto's party won a general election in February 2008 with her supporters angered by conflicting accounts of how she died and who was responsible.
They cast doubt on a Pakistani probe into her death, criticised authorities for hosing down the scene of the attack within minutes -- allegedly destroying evidence -- and questioning whether she was killed by a gunshot or the blast.
"The responsibility for investigating the crime and prosecuting the perpetrators remains with Pakistani authorities," said Munoz, who is Chile's ambassador to the United Nations.
The commission on Friday visited the site of the gun and suicide attack.
Munoz said new leads obtained by the Pakistani authorities would help in their inquiries and appealed for help from the general public.
"We have been informed that Pakistani government has new leads into the investigations... the fresh information will contribute to our task."
The UN official said an email address and telephone number would be provided should anyone wish to come forward and provide information.
"We cannot be fully successful without the help of the people of Pakistan... We are approaching the work in a transparent and open-minded manner to clearly establish the truth surrounding this tragedy," said Munoz.
"Our work will be guided by objectivity and independence."
Munoz said the commission might approach a third country, which he did not name, to secure more information.
British detectives ruled that Bhutto died from the force of a suicide bomb and not gunfire, backing the Pakistani government's controversial account.
Bhutto, the first of whose two stints as prime minister began in 1988, wrote in her autobiography of warnings that four suicide squads -- including one sent by a son of Osama bin Laden -- were after her.
She also repeatedly accused a cabal of senior Pakistani intelligence and government officials of plotting to kill her, notably in an attack that killed 139 people in Karachi on October 18, 2007 when she returned from exile.
Then-president Pervez Musharraf and the US Central Intelligence Agency blamed Baitullah Mehsud, an Al-Qaeda-linked warlord based in Pakistan's tribal region bordering Afghanistan, for masterminding her killing.
The commission will submit a report to UN chief Ban Ki-moon by the of December, which will be shared with the Pakistani government and the UN Security Council.