UNITED NATIONS - The United Nations has not so far received a formal letter from Pakistan seeking a U.N. investigation into the assassination of PPP leader Benazir Bhutto, a world body spokesman said Friday. "We have not received the letter," Spokesman Brenden Verma said, while commenting on PPP Co-Chairman Asif Ali Zardari's statement at a press conference in Islamabad that a request has already been sent. The UN has already said that as soon as a formal request from the government of Pakistan is received by Secretary-General Ban Ki-moon, he would transmit it to the Securuty Council for action. Ms. Bhutto's was assassinated in a gun-suicide attack late last year in  Rawalpindi. The previous government had blamed Pakistani Taliban leader Baitullah Mehsud of being behind the conspiracy to kill Bhutto, but he has issued a denial from his stronghold in the South Waziristan.. The government earlier invited help from Britain's Scotland Yard to determine how Bhutto was killed, though the British police were not asked to investigate who killed her. Scotland Yard backed up the government's earlier conclusion that Bhutto smashed her head against her vehicle during the attack. The PPP harbors deep suspicions over the government's findings and doubts whether Mehsud was the real culprit. Ms. Bhutto had long feared members of the Pakistani establishment would plot to kill her after she returned from years of self-exile in October.