Jia, a columnist active on political issues with some 85,000 followers on Twitter, was scheduled to depart from Beijing airport March 15 to attend a speaking event and renew his work permit.
He last spoke with his wife on the phone upon arrival at the airport but has not been in contact since, his lawyer Yan Xin said. There was no indication he made it to Hong Kong.
Yan said that Jia had been taken for "criminal investigation," but it wasn't clear whether Jia was detained as a suspect or to assist in an investigation. He added that he thought the investigation was linked to a recently published open letter critical of President Xi Jinping.
Yan said Friday that Jia was not the anonymous author of the article.
Chinese authorities have so far not publicly commented on Jia's case. The public security bureau of the Beijing Capital International Airport told the Committee to Protect Journalists that it had no information about him.
Dear Xi Jinping
Some were quick to link Jia's sudden disappearance to an online open letter addressed to Xi this month, which said Xi's heavy-handed approach in consolidating power has "caused unprecedented problems and crises" for the nation.
Yan said Jia did not write the letter, which was first published on March 4 by Canyu.org, a Chinese-language website focusing on China's human rights issues. It was then quickly republished by several other websites including Watching.cn, an online news outlet backed by the Chinese government.
Watching has since removed the letter from its website, but CNN was able to locate and verify its cached version. Columnist Mo Zhixu, a friend of Jia Jia's, told CNN that Jia Jia alerted Watching editor Ouyang Hongliang about the letter and suggested it be taken down due to security concerns.
Friends and contacts of Jia Jia that CNN spoke to said they suspected the journalist's disappearance might be connected to his contact with the Watching editor about the letter.
The letter, written under the alias "a loyal Communist Party member," briefly applauded Xi's accomplishments, including an anti-corruption campaign and deepening economic reforms, before turning to sharp criticisms of the leader.
Politically, the letter said, Xi has weakened the independence of China's political organs and caused confusion within all levels of Communist Party government officials in terms of responsibilities and decision making.
Diplomatically, Xi's foreign policy has allowed for North Korea's successful testings of nuclear and hydrogen bombs, the United States' pivot back to Asia, and an anti-China united front formed by South Korea, Japan and Southeast Asian nations, the letter said.
Economically, the letter said, policies formulated by Xi's central economic leadership team have led to turbulence inside China's stock and property markets, huge losses of civilian wealth, excessive energy outages, massive unemployment and currency devaluation.
Ideologically, Xi's permission for a cult of personality has caused fear among those who experienced the nation's infamous Cultural Revolution, according to the letter.
"We ask you that for the sake of the party's prosperity, for the sake of the nation's long-term stability, and for the sake of you and your family's safety, to resign from all your duties for the party and the nation, and let the party and the Chinese people select another capable person who can lead us to actively advance to the future," the letter concludes.
Some say Jia's detention -- if he is being detained -- would be another example of tightened media censorship under Xi's leadership, citing his recent tour of Chinese state media headquarters.
"We are deeply concerned by Chinese journalist Jia Jia's disappearance," Bob Dietz, the Committee to Protect Journalists' Asia program coordinator, said in a statement. "If he is in police custody, officials must disclose where they are holding him and why. If anyone else knows where he is, they should step forward and clarify this worrisome mystery."
Xiao Han, a Chinese blogger, wrote on Friday in a post titled "Jia Jia's disappearance and the super gang," offering his take on the public's suspicion of Chinese authorities' role in Jia's disappearance.
"This is because that for a long time, the Chinese people have never had the freedom to be fear-free. In a country where police authority is subject to no justice review, occurrence of any type of state terrorism is not surprising," Xiao wrote.
"So, what kind of super gang has such capability? It has to be the national team."