Security officers arrested seven students from an Islamic school hours after the blast Friday, which apparently targeted former Interior Minister Aftab Khan Sherpao, who had been deeply involved in Pakistan's fight against militants linked to the Taliban and al-Qaida.
The bomber struck a mosque inside the politician's residential compound in the village of Sherpao during prayers for the Islamic holy day of Eid al-Adha, leaving a scene of splattered blood, body parts, prayer caps and shoes.
Sherpao escaped harm, but one of his sons was wounded and at least 50 people were killed. The former minister survived an earlier suicide attack at a rally in the nearby town of Charsadda eight months ago that killed 28 people. Officials blamed al-Qaida for that attack.
Suspicion for the blast was expected to focus on the pro-Taliban or al-Qaida militants active in northwest Pakistan, near the Afghan border. The attack also deepened the sense of uncertainty in Pakistan ahead of Jan. 8 parliamentary elections, which Sherpao, as head of the Pakistan Peoples Party-Sherpao, is contesting.
Police collected pieces of clothing, shoes, prayer mats and two severed legs of a man from the scene of Friday's bombing, and investigators were examining them for clues to identify the bomber, an official involved in the probe said.
"We are looking at how it happened and who did it," he said, speaking on condition of anonymity because he was not authorized to talk to the media.
The raid of the Islamic school in the nearby village of Turangzai was carried out based on intelligence information, a second police official said, speaking on condition of anonymity because of the sensitivity of the subject.
The officer indicated the arrests could be linked to the bombing, but he would not elaborate. Of the seven students arrested, three were Afghans.
Pakistani President Pervez Musharraf condemned the blast and directed security and intelligence agencies to track down the masterminds, the state-run Associated Press of Pakistan reported. U.N. Secretary-General Ban Ki-moon also condemned the attack.
The bomber was in a row of worshippers when he detonated the explosive, provincial police chief Sharif Virk said Friday. Witnesses said the dead included police officers guarding Sherpao, who was praying in the mosque's front row at the time of the attack.
As interior minister, Sherpao oversaw one of Pakistan's civilian spy agencies, police and paramilitary forces involved in operations against militants along the Afghan border.
He was a longtime supporter of former Prime Minister Benazir Bhutto's Pakistan People's Party before defecting and joining the government after the last parliamentary elections in 2002. He left office last month as a caretaker government took over ahead of the January elections.
Taliban and al-Qaida fighters have extended their influence over tracts of Pakistan's volatile northwest in the past two years and in recent months have launched numerous suicide attacks, usually targeting security forces and their families.
The army says the most recent attacks could be retaliation for a military operation against militants in the Swat valley, where it claims to have killed about 300 militants since last month.
The violence came as Pakistan struggled to emerge from months of political turmoil.
Musharraf recently declared emergency rule for six weeks a move he said was necessary to combat rising Islamic extremism, but was widely seen as a ploy to prolong his own presidency. Thousands of his opponents were rounded up and Supreme Court justices fired.
Sadaqat Jan contributed to this report from Islamabad, Pakistan.