Pakistani police have rounded up more than a dozen people for questioning over last week's suicide bombings in the southern port city of Karachi, officials said on Wednesday.
"We have held 18 people for questioning," an investigator said on condition of anonymity, adding some of them were among the injured and policemen deployed for the security of Bhutto's motorcade.
He also claimed the decapitated head of the suspected suicide bomber found on the scene had been identified, but gave no further details.
Two suicide bombers attacked a procession last week that Bhutto was leading after her arrival from Dubai to end an eight-year self-imposed exile, killing 140 people and injuring more than 500.
Security agencies have reported that high-explosive material was used with Russian devices in the suicide belts.
"Ball bearings were embedded in the explosive belt for more lethal effect on men and material," said a technical report prepared by Special Investigation Groups of Federal Investigation Agency.
A Pakistan army team was aiding the police investigators, especially in the forensic tests of the remains of the suicide attackers.
Bhutto, two-time prime minister and opposition leader, has expressed mistrust about the country's security agencies and called for an independent inquiry led by foreign investigators from the US and Britain.
But the government has rejected the demand.
"How would Americans help us when they have failed in finding Al Qaeda leader Osama bin Laden and Taliban leader Mullah Omar," said Sheikh Rashid, Railway Minister and a close confidant of President General Pervez Musharraf.