The suicide bomber who tried to assassinate former Pakistani premier Benazir Bhutto [Images] had tried to break through her security cordon by bringing a box of cotton buds that she had requested, but was turned away twice.

The Sunday Telegraph, quoting a source close to investigations into the attack which killed 165 people, said having penetrated an outer ring of security on his third attempt, the bomber is thought to have thrown a grenade as a diversion before detonating a device made of C4 plastic explosive and ball bearings.

The bomber's severed head, which remained intact, was recognised by police as the man they twice turned away, the report said.

After surviving the attack, Bhutto, who earlier said attempts on her life could come from within the government, told the paper that she plans to purge intelligence services of hundreds of rogue agents suspected of supporting Islamic terrorism if she returns to power.

"I have strong reservations about some of the people still operating within the intelligence services, and we need reforms to get rid of them," she said.

"We need a security service that is professional in its approach, which rises above ties of religious or political sentiment," Benazir said.

In remarks that puts her in direct confrontation with the nation's most powerful institutions, Bhutto, who returned home last week after eight years in self-exile, said Pakistan's security agencies had to become "professional" outfits free from political agendas.