The detective leading the probe into the homicide attack on opposition leader Benazir Bhutto withdrew from the case Wednesday after she accused him of being involved in the torture of her husband in 1999, a senior official said.

Ghulam Muhammad Mohtarem, the home secretary of Sindh province, said the government would form a new team of investigators to probe the attack in Karachi last week, when a pair of explosions rocked Bhutto's homecoming parade after eight years in exile, spewing metal shards through a sea of supporters and killing at least 136 people.

The attack has raised fears about Pakistan's stability amid a rising Islamic militancy — fears further underlined Wednesday when the government announced it had sent 2,500 troops into a remote valley to combat a militant cleric who calls for Taliban-style rule and holy war against Pakistani authorities

But the attempt at intimidation appeared to fall flat, as some 6,000 supporters of the cleric, Maulana Fazlullah, gathered in a schoolyard to hear him speak just a few miles (kilometers) from where the soldiers deployed.

Fazlullah, wearing a black turban and with long, flowing hair, addressed the crowd from the back of a truck, a dozen armed men deployed around him as bodyguards.

"The government has made a declaration of war," he said, according to a local journalist who witnessed the scene. "Is it a crime to sit in the home of Allah and to study the Quran?"

The paramilitary troops deployed Tuesday across Swat, a mountain valley popular with tourists until violence flared there this summer, army spokesman Maj. Gen. Waheed Arshad said.

Militants responded by detonating a remote-controlled bomb near a convoy late Tuesday, wounding four soldiers. Arshad said security forces had detained seven suspects.

The army already sent regular troops into Swat, which lies about 50 kilometers (30 miles) north of the city of Peshawar in July as part of a crackdown on militancy spreading across the region.

The deployment prompted a string of bombings and homicide attacks on security forces.

Fazlullah is the leader of Tehrik Nifaz-e-Sharia Mohammed, or TNSM, a pro-Taliban militant group which sent thousands of volunteers to Afghanistan to during the U.S.-led invasion in 2001. Pakistan banned the group and jailed TNSM's founder, Fazlullah's father-in-law, in 2002.

But the group re-emerged this year in Swat and Malakand, another impoverished, conservative region next to the Afghan border.

As well as marshaling armed militants and enforcing Islamic law, Fazlullah has reportedly used an FM radio station to campaign against girls' education and denounce a recent polio vaccination program as a Western plot to sterilize Muslim children.

Bhutto has blamed Islamic militants for the attack on her, but has also accused elements in the government and the security services of complicity, demanding international experts be called in to help in the investigation.

She had specifically objected to Manzar Mughal, a senior investigator in the Sindh province police force, claiming he had been present while her husband, Asif Ali Zardari, was tortured in custody on corruption charges in 1999.

Mohtarem said the provincial government had no doubt about Mughal's competency and professionalism, but said he had decided to withdraw to protect the investigation from accusations of bias.

"The investigation team will be formed anew after Manzur Mughal disassociated himself from the investigation in view of the objections raised by Benazir Bhutto on the chief investigator's credentials," he said.

"We know what we're doing. We don't need assistance," Prime Minister Shaukat Aziz told reporters later, adding that: Mughal's withdrawal "doesn't change the very fact that we have a whole process looking into such cases."

Investigators are holding at least 15 people for questioning in the blast, an investigator said.

Some of the people being held were wounded in the attack and picked up from local hospitals. None are currently being treated as suspects, a police investigator said, speaking on condition of anonymity because he was not authorized to speak to the media.

A second police investigator confirmed a number of people were being questioned.

Authorities have said the attack was likely carried out by two homicide bombers, and have released a picture showing the head of one of the attackers.

Bhutto, whose two governments between 1988 and 1996 were toppled amid allegations of corruption and mismanagement, has returned to contest parliamentary elections due in January, after months of talks with President Gen. Pervez Musharraf that could see them working side-by-side in the next government.

With encouragement from Washington, both are urging voters to support moderates willing to battle the rising religious extremism.

Aziz, meanwhile, played down concerns that security issues would place so many restrictions on the campaign that the elections would not be free and fair.

Officials initially proposed a ban on public rallies, but have backed down amid opposition protests. The government is now working on a code of conduct with political parties to make large gatherings easier to secure.

"We'll allow full campaigning. There is a misconception that we in some way may be restricting activity," Aziz said.