Authorities questioned a 15-year-old boy who reportedly confessed to joining a team that killed Pakistan opposition leader Benazir Bhutto, while helicopter gunships attacked suspected militant positions close to the Afghan border, officials and witnesses said Sunday.

A villager said two civilians were killed in the attacks Sunday in the South Waziristan region, where a spike in fighting in recent days has killed about 100 people, most of them militants. But military spokesman Maj. Gen. Athar Abbas said there were no reports of casualties.

The arrest of the teenager in the town of Dera Ismail Khan in Pakistan's North West Frontier province could be the first break in the investigation into Bhutto's killing on Dec. 27 in a gun and suicide bomb attack.

A senior intelligence official said the boy was arrested Thursday along with another militant suspect.

He told investigators that his five-person squad was dispatched to Rawalpindi — where Bhutto was killed — by Baitullah Mehsud, a militant leader with strong ties to Al Qaeda and an alliance with the Taliban in nearby Afghanistan. The official asked not to be named because he was not authorized to speak to the media.

Authorities have already accused Mehsud, who is believed to be hiding in South Waziristan, of organizing the killing.

Interior Secretary Kamal Shah also said the boy had confessed to involvement in the slaying. Both of the detainees were being questioned in an attempt to corroborate the confession, he said.

Bhutto's assassination triggered days of unrest that left 40 dead and thrust Pakistan into a deep political crisis at a time of surging attacks by Al Qaeda and Taliban militants. The violence came as the country prepares for Feb. 18 polls that many predict will weaken President Pervez Musharraf's grip on power.

Bhutto, who had returned to Pakistan in October after spending nearly eight years in exile, had vowed to support tough military measures against Islamic militants who have used the border areas as staging points for infiltration into Afghanistan.

Meanwhile, the country's Shiite minority held processions and prayers to mark the festival of Ashoura, which in the past has been marred by attacks from Sunni extremists, who regard Shiites as heretics.

Tens of thousands marched and beat their bare backs with chains and blades, bloodying themselves in a sign of penitence. They said the threat of violence did not worry them.

"The procession is not something that could be curbed through fear of death," said Qaiser Abbas Zaidi, a retired civil servant in Rawalpindi, a garrison town close to the capital, Islamabad. "People are slashing their heads with knifes and beating their chests in mourning. It means they are ready for death."

On Thursday, 11 people died and 20 were injured in a suicide attack on a Shiite mosque in the northern city of Peshawar.

In the far southern city of Karachi, the police chief said officers detained five men on Saturday with explosives, detonators and a small quantity of cyanide intended for attacks on Ashoura.

"With these arrests we have foiled major attacks," police chief Azhar Farouqi said, adding that the militants may have wanted to put the cyanide into the municipal water supply.

The rising violence has undermined the popularity of Musharraf, a key ally of the U.S. in its war on terrorism.

He left Sunday for Europe on an eight-day trip to meet leaders and attend the World Economic Forum in Switzerland. He said the trip was aimed at "improving Pakistan's image and removing certain misperceptions," but did not elaborate, according to the state-run news agency.