Police were questioning three men over the attack on former Pakistan premier Benazir Bhutto's motorcade with investigations revealing evidence suggesting that the suicide bomber was not alone and was accompanied by four or five accomplices.
Three men from Punjab province were taken in for questioning on Saturday night after being linked to a car from which an attacker allegedly threw a grenade at Bhutto's motorcade.
Investigators are also focussing on a second severed and badly mutilated head found on a tree a short distance from the site of the two blasts that killed 165 people and injured hundreds more.
The probe had initially centred around a severed head found at the site of the explosions.
Police had released photos of this head, believed to be of a local, clean-shaven man in his twenties.
A senior security official told the Dawn newspaper on condition of anonymity that the suicide bomber had been part of a suspicious-looking group of men, some of whom had been seen holding sticks.
This group had been spotted by a policeman standing under a flyover in the Karsaz area of Karachi, where Bhutto's armoured truck was targeted by the suicide bomber who was supposedly armed with a device containing about 15 kg of explosives and thousands of pellets that caused widespread devastation and mayhem.
Before the policeman could approach the group, the men melted into the huge crowd.
"These men were distinguishable from the rest of the crowd because they were wearing white salwar kameez and did not appear to be enjoying the music and joy of the occasion," the security official was quoted as saying.