A former lawyer for the Pakistani doctor who helped the US find Osama Bin Laden has been killed by gunmen in the north-western city of Peshawar.
Samiullah Afridi was on his way home on Tuesday when unidentified assailants fired at his car. He later died.
In 2013 he fled Pakistan following threats by militants. After returning last year he told the BBC that he had stopped working on the doctor's case.
At least two Taliban splinter groups have claimed the killing.
Dr Shakil Afridi, who is no relation to Samiullah, is appealing against a 33-year sentence after being found guilty of collaborating with the Lashkar-e-Islam militant group in his native Khyber tribal region.
But many saw his sentence as punishment for aiding the CIA in their hunt for Bin Laden.
He helped organise a fake vaccination programme aimed at gathering DNA samples from some of the residents at a compound where the former al-Qaeda chief was found.
The case damaged relations between the US and Pakistan, with American forces raiding the compound without permission from Islamabad.
Samiullah Afridi is believed to have returned from the Middle East a year ago having left Pakistan for a few months.
He said he had received frequent death threats after taking up Dr Afridi's case.
"I took the case on humanitarian grounds, but now I have to look for my own life, it is more important," he said last year.