The Afghan government says the killer of Burhanuddin Rabbani, its negotiator with the Taliban, was a Pakistani citizen.
Mr Rabbani - who was attempting to mediate between the Afghan government and the Taliban rebels - was assassinated by a suicide bomber on 20 September by a suicide attacker who purported to be a Taliban peace envoy.
The BBC reports Islamabad has denied any role in Rabbani's death.
Both Afghanistan and the United States have accused the ISI, Pakistan's intelligence agency, of supporting insurgent groups.
After the killing, President Hamid Karzai said his government would no longer hold peace talks with the Taliban, but would focus on dialogue with Pakistan.