The suspect, 15-year-old Aitezaz Shah, was arrested from the northwestern city of Dera Ismail Khan on Friday while planning a suicide bombing over the Muslim festival of Ashura, they said on condition of anonymity. Shah told interrogators he had been part of a back-up team of three bombers who were tasked with killing former premier Bhutto if the original December 27 attack by two men had failed, the officials added. Interior ministry spokesman Iqbal Cheema did not confirm the arrest. "It is not in my knowledge so far," Cheema told AFP. Bhutto was assassinated in a gun and suicide bomb attack at an election rally in Rawalpindi. Pakistan`s government and the CIA have blamed Al-Qaeda and tribal warlord Baitullah Mehsud for her killing. Shah, originally from the southern city of Karachi, went for training last year at a camp run by one of Mehsud`s commanders in the tribal border region of Waziristan, the security officials quoted him as telling investigators. He allegedly said the attackers in the team that killed Bhutto were called Bilal and Ikramullah -- the same names mentioned in an alleged telephone conversation between Mehsud and another militant the day after Bhutto`s death. The tape was released the day after her killing by Pakistan`s interior ministry. Shah`s whereabouts at the time of the attack were not immediately clear. One security official said he was in Rawalpindi, the city where Bhutto was killed, while another said he was in the tribal area of Waziristan. "The suspect was not in Rawalpindi at the time of attack on Bhutto. The boy told interrogators that he was in South Waziristan," a top police official quoted him as telling the interrogators. One of the officials said Shah was arrested during a security check when he arrived in Dera Ismail Khan by taxi from the North Waziristan tribal area, which borders Afghanistan. He allegedly told officials that he came to collect a suicide jacket for an attack at the US consulate in Karachi but the programme was changed because of tight security for Ashura, which takes place on Sunday. Instead he was ordered to launch an attack during an Ashura procession by the minority Shiite sect on Sunday, the officials said. A spokesman for Taliban militants in Pakistan rejected the allegations, saying that the boy arrested in Dera Ismail Khan had no link with the group. "We are not involved in the attack and this information is meant to defame us," the spokesman Mohammad Omar said in telephone calls to reporters in Peshawar. "He is not our man and we know nothing about him" Omar said adding that its purpose was to give credence to a CIA report about involvement of Baitullah Mehsud in Bhutto`s assassination