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 said.
Bhutto was assassinated in a gun and suicide bomb attack at an election rally in Rawalpindi. The government and US 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.