His abduction came days after he photographed shrapnel from a Hellfire missile, allegedly fired by an unmanned American warplane that killed a wanted Egyptian al-Qaida figure, Hamza Rabia, in Mir Ali.
Khan's widely published photograph contradicted a Pakistani government claim that Rabia had died in a bomb-making accident.
No one has claimed responsibility for abducting Khan, but his relatives blame Pakistan's top spy agency. The government denies that the Inter-Services Intelligence, or ISI, played a role, and authorities have blamed Islamic militants.
Information Minister Mohammed Ali Durrani told the journalists that a High Court judge will lead a probe into Khan's abduction and killing, the AP reports.
Similar protests were being held across Pakistan to condemn Khan's killing, Usmani said.
Khan worked for Pakistan's Urdu-language daily Ausaf and the European Pressphoto Agency.
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