Investigators probing the terror attack on former Pakistan premier Benazir Bhutto's [Images] motorcade in Karachi are focussing on the severed head of the suspected suicide bomber but avoided blaming any particular militant group for the twin blasts that killed nearly 165 people.

The head, found shortly after the failed assassination bid on Friday, has been used to prepare a sketch that was released by police as part of their efforts to obtain information about the persons behind the attack which has caused widespread shock and anger in this port city.

Investigators and explosives experts have scoured the site of the blasts in Karsaz area and collected samples. City police chief Azhar Farooqi said the probe was progressing in the 'right direction' but avoided attributing the attack to any particular group of militants.

There was also speculation that the investigation might centre round Bhutto's allegations that three senior government functionaries were behind the worst terrorist attack in Pakistan.

Bhutto had said on Friday that she had named these three persons in a letter written to President Pervez Musharraf [Images] on October 16 but refused to identify them in public.

Bhutto has also claimed that one of these persons is being 'watched' by the authorities. Media reports on Saturday suggested that senior intelligence and army officials were among the three persons.

The administration of Sindh province has offered a reward of Rs 50 lakh for information on those involved in the attack while police have registered a case against unidentified persons at Bahadaurabad police station under the Anti-Terrorism Act.

Interior Minister Aftab Sherpao said it was not possible to say that the blasts were the handiwork of Al Qaeda [Images], accused by Bhutto of planning to kill her.

"Investigations are still going on. So far we cannot say it was done by Al Qaeda," he said.

Doctors at the Jinnah Postgraduate Medical Centre in Karachi are reconstructing the mangled face of the suspected suicide bomber, who appeared in his 20's, and samples from the severed head have been sent for DNA tests, Dawn News channel reported.

Meanwhile, a leading daily claimed that four bombers had arrived in Karachi from Waziristan recently to target Bhutto when she flew into the city on October 18. The newspaper quoted sources as saying that home department and police of Sindh province had been informed about these attackers.

It claimed there was another suicide bomber at the scene of Friday's attack but 'slipped away.'

Police officials in Sindh have said they had informed Bhutto's Pakistan People's Party about the threat to her from jihadi groups linked to the Taliban and Al Qaeda and advised the party to curtail her appearance in public.

The officials have also blamed PPP workers for not adhering to security protocols devised to protect the former prime minister.

Musharraf, who condemned the blasts during a telephone conversation with Bhutto yesterday and assured that the culprits would be brought to justice, has sought a report on the attack from intelligence agencies by Sunday.

As Bhutto's massive motorcade inched its way through Karachi early on Friday, a grenade was lobbed near the armoured truck carrying the former premier. A suicide bomber carrying almost 15 kg of explosives then blew himself up near the truck.