Police in Vietnam’s Ho Chi Minh City have arrested activist Phan Tat Thanh, accusing him for criticizing the government’s response to “Chinese aggression” in the South China Sea, his father told Radio Free Asia.

Thanh, 37, also known as “Black Aaron,” often posted online about the contentious area in the sea where Hanoi, Beijing and others have competing territorial claims.

Netizens told Thanh’s father, who requested anonymity for security reasons, that Thanh had gone missing on July 5.

The police issued a prosecution document on July 13, and on July 15 they searched his home and copied data from his computer.

In 2010, Thanh staged a protest in front of China’s Embassy in Bangkok because anti-China demonstrations in Vietnam by that time were being suppressed.

In addition to anti-China posts, Thanh had written posts and comments about human rights violations, environmental pollution, systematic corruption, and issues of major concern in Vietnam.