Pakistan ? An Islamic militant who is accused of helping carry out the assassination of former Prime Minister Benazir Bhutto wanted to avenge the death of a friend in a military attack last year, a senior police officer said Wednesday.

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 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.

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-Qaida-linked extremist in northwest Pakistan.