Bhutto was killed in a shooting and bombing attack at an opposition rally in Rawalpindi on December 27, 2007, that also took the lives of more than two dozen bystanders. The exact cause of her death is still disputed.
Party spokespersons said Asif Ali Zardari, Bhutto's widower who has become a PPP co-chairman, addressed the letter to UN Secretary General Ban Ki-moon requesting that an investigation commission be set up to look into the case.
Copies of the letter were also submitted to the embassies of the five UN Security Council permanent members.
The government had invited British investigators to assist the probe, but the PPP criticized the authorities for their refusal to forward their petition to the UN despite their plea.
After the assassination of Bhutto, who twice served as the country's prime minister and was to run in parliamentary elections on January 8 as PPP leader, violence escalated further in the country, with dozens killed in bomb attacks and riots. The government had to postpone the elections until February 18.
