Mr Jia is said to have been detained as part of a high-profile inquiry into a letter urging Mr Xi to quit.
The letter appeared briefly on a website linked to China`s government.
Signed by "Loyal Communist Party Members" and dated March 2016, it criticised Mr Xi for various political, economic and diplomatic decisions, as well as for creating a "personality cult".
"We don`t think Comrade Xi Jinping has the ability to lead the party and the country into the future," it said.
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Mr Jia`s family deny he has anything to do with it.
Friends told media outlets they believed Mr Jia disappeared because he warned an editor friend, Ouyang Hongliang, after the letter was published on Mr Ouyang`s news site Watching, also known as Wujie News.
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