(SHERIN ZADA ~ Associated Press)
The teen also confessed to taking part in a plot to attack Shiites during Ashoura, even as police in Pakistan's far south said they had foiled suicide attacks planned for the Shiite Muslim festival.
In Karachi, police chief Azhar Farouqi said officers detained five men who were in possession of explosives, detonators and a small quantity of cyanide intended for attacks on this week's Ashoura processions.
"With these arrests we have foiled major attacks," Farouqi said.
The intelligence official said the 15-year-old told investigators that the five-person squad was dispatched to Rawalpindi, where Bhutto was killed, by Baitullah Mehsud. Mehsud is a militant leader with strong ties to al-Qaida and an alliance with the Taliban in nearby Afghanistan.
The senior official who spoke on condition of anonymity because he was not authorized to speak to the media, said the teen was arrested Thursday and was involved in a plot to attack Shiites during an Ashoura festival on Sunday.
Pakistan's Sunni extremists, who regard Shiites as heretics, often attack the community during Ashoura. On Thursday, 11 people died and 20 were injured in a suicide attack on a Shiite mosque in Peshawar, the capital of North West Frontier Province.
A senior district police officer in Dera Ismail Khan, a town 168 miles southwest of Islamabad, confirmed the teen's arrest there and said the suspect made "a sensational disclosure."
The officer also asked not to be named because of the sensitivity of the matter.
In the capital, Islamabad, Interior Ministry spokesman Jawed Iqbal Cheema said he had no information about any arrests in the border area, or about any new developments in the Bhutto case.
Maulvi Mohammed Umar, a purported spokesman for Mehsud, denied his group had links with the teen, and said he had not been dispatched by Mehsud to kill Bhutto.
"It is just a government propaganda," he said. "We have already clarified that we are not involved in the attack on Benazir Bhutto."
Bhutto, a former prime minister, died on Dec. 27 when one member of the squad, whom the teen allegedly identified as Bilal, fired at her and detonated an explosive vest as Bhutto was leaving a campaign rally in Rawalpindi, which is adjacent to Islamabad. The blast killed at least 20 other people and wounded scores more.
The death of Bhutto, Pakistan's most popular opposition leader, threw the country into turmoil and triggered riots that left more than 40 people dead. It forced the government of President Pervez Musharraf to delay by six weeks parliamentary elections that had been set for Jan. 8.
In Washington, U.S. intelligence officials have concluded that Mehsud, who heads a network of armed groups in the lawless region along Pakistan's border with Afghanistan, organized the attack on Bhutto as part of a campaign of assassinations of Pakistani officials and suicide bombings in the country.
Bhutto had returned to Pakistan in October after spending nearly eight years in exile following the military coup in which the U.S.-allied Musharraf seized power in 1999.
Bhutto had vowed to support tough military measures against Islamic militants who have used the border areas as staging points for infiltration into Afghanistan.
On Friday, the military said that up to 90 Islamic militants aligned with Mehsud had died in clashes with Pakistani troops in South Waziristan.
The intensifying combat highlighted the deteriorating security in the region.
On Saturday, Mehsud's spokesman denied that the army had killed 90 of their people.
"The army is killing innocent people in our area, and we will avenge it," he said.
In a new show of strength, hundreds of Mehsud's fighters mounted attacks in the past several days on the two forts in the region, exposing the Pakistani military's weak grip over the area.
On Saturday, the army said it had arrested 50 suspected militants in the area.