Pakistani investigators now suspect two suicide bombers were responsible for Thursday's attack on the procession of former Prime Minister Benazir Bhutto.
Officials earlier thought only one suicide bomber was involved in the explosions that killed nearly 140 people and wounded hundreds of others in Karachi. Authorities believed the first blast was caused by a grenade.
On Monday, investigators in Karachi said they found the severed head of a second suspected suicide bomber, and they are reconstructing the face to identify the person.
Ms. Bhutto visited the tomb of Pakistan's founder Mohammad Ali Jinnah to renew her call for an independent international investigation into the blasts.
Pakistan's Interior Minister Aftab Sherpao rejected that request, saying Pakistan's law enforcement agencies could handle the probe.
Also Monday, Pakistan's opposition parties denounced a government proposal to ban large political rallies and processions in response to last week's bombings.
Members of Ms. Bhutto's Pakistan People's Party say rallies are important to the election process.
Former Pakistani Prime Minister Nawaz Sharif and members of Islamist groups also expressed concern about the proposed ban. They say the government does not want opposition leaders reaching out to their voters.
Critics of the proposed ban such as PPP ally Senator Babar Awan say the measure has nothing to do with improving security in the wake of the bombings.
The United Nations Security Council issued a statement Monday condemning the bombings and urging the international community to work together to bring to justice those responsible for the terrorist acts.
