Husnain Gul and his cousin, identified only as Rafaqat, were arrested last week in the Dec. 27 death of Bhutto, who was killed in a gun and suicide bomb attack at the end of a political rally in Rawalpindi.
The senior policeman leading the murder investigation, Chaudhry Abdul Majeed, told reporters that Gul began preparing for the attack in November.
His friend had been killed in military operations against Islamic militants holed up inside Islamabad's Lal Masjid, or Red Mosque, in July. About 100 people died in the siege.
"Their motive for attacking Bhutto was that she was coming to Pakistan at the behest of a foreign power," Majeed said.
Majeed would not speculate on the mastermind of the assassination. U.S. and Pakistani officials suspect the attack was ordered by Baitullah Mehsud, an al Qaeda-linked extremist in northwest Pakistan.
Four people, including a 15-year-old boy, have been arrested in the probe. Majeed said a fifth suspect is being sought but he refused to elaborate.
Majeed said that Gul and his cousin told investigators details of the plot against Bhutto. Based on that testimony, Majeed said police have learned that two assailants were stationed at a pair of gates to the park where thousands of Bhutto supporters had gathered.
Majeed said the two suicide bombers spent the night at Gul's home in Rawalpindi on the eve of the attack. Gul also drove the two bombers to the public park where the attack occurred, Majeed said.
The primary assailant was identified only as Saeed, who was using the alias Bilal.
Majeed said Bilal opened fire with a pistol and then detonated explosives hidden beneath his clothing, fatally injuring Bhutto.
The other assailant, identified only as Ikramullah, was to have attacked Bhutto if she had escaped the first blast. With the first bomb proving fatal, Ikramullah left Rawalpindi the next morning, Majeed said, without saying where.