The teenager admitted to investigators that he was one of five suicide bombers sent by Baitullah Mehsud, a pro-Taliban leader in Pakistan, to assassinate Bhutto in the city of Rawalpindi, the sources said.
Shah said that two of the attackers, Akram and Bilal, were to target Bhutto first. If they failed the other three were to carry out the operation.
He said Bilal killed Bhutto by shooting her and detonating an explosive vest as she was leaving an election campaign rally on December 27.
The explosion killed at least 20 other people and wounded scores more.
The whereabouts of the two other members of the group are unknown.
Shah returned to the NWFP after the operation and was assigned to a suicide mission on an unspecified foreign consulate in Karachi.
Mehsud allegedly has strong ties to al-Qaeda and an alliance with Taliban.
He heads a network in South Waziristan and has been blamed for an organised campaign of assassinations of Pakistani officials and suicide bombings in the country.
The Pakistani government and CIA have claimed that Pakistan's Taliban was behind the assassination Bhutto.
The Washington Post
first reported the CIA's conclusion on Friday, in an interview with CIA director Michael Hayden.
"This was done by that network around Baitullah Mehsud. We have no reason to question that," Hayden said.
CIA officials said Mehsud, believed to be in his early 30s, is a "committed jihadist" who recruits and trains suicide operatives for the Taliban and al-Qaida.
An army official said on Saturday that troops had captured 50 pro-Taliban fighters in an operation near the Afghan border, an area where Mehsud's fighters are known to operate.
Soldiers also recovered the bodies of 10 fighters from Chaghmalai area of South Waziristan, Major-General Athar Abbas, the chief military spokesman, said.
Security forces separately launched a search operation in Ladha and arrested 10 fighters, including some of their local commanders, Abbas said.
Pakistani soldiers on Friday fought off a "large number" of fighters who had attacked the military at a fort in Ladha, killing 50 to 60 of the attackers, the army said in a statement.
The same day, Pakistani forces claimed to have killed up to 90 pro-Taliban fighters in two separate battles in South Waziristan.