Explosives experts of Pakistani Special Investigation Group have recovered the head of a suicide bomber from the rooftop of the Doctors' Hostel adjacent to the bustling Melody Chowk blast site.

A suicide bomber on Sunday evening ripped through a police contingent at Melody Chowk, killing at least 20 people, including 16 personnel of Islamabad and the Punjab police and injured more than 53 people, including 30 cops.

The investigators recovered fingers, including thumb, of the attacker that would assist in his identification, local media reports said on Tuesday.

The suicide bomber struck the police group from behind with all police personnel sustaining injuries on their back.

"Certainly, the attacker was waiting in a tea bar for a suitable time to strike," officials said.

"The terrorist used six to eight kilogramme power explosives material and it was a locally-made device," an explosives expert said, adding small pallets were used in the explosive belt.

"The face of the suicide attacker is 50 per cent safe but it can be refaced easily," a source said, adding lips were intact but eyes have blown out. The skull and hair were not damaged, he said.

Cranium of the suicide bomber has been sent for DNA test and his fingers to national database registration authority for his identification.

According to security personnel, investigation agencies were also reviewing list of those persons who met former Lal Mosque administrator Maulana Abdul Aziz in jail during the last three weeks.