The 53-year-old Ai joins a group of artists who have recently been detained or jailed as part of a widespread effort on the part of the Chinese government to rein in critical voices across traditional and new media.

While state media originally reported that Ai was arrested at the airport because his departure procedures were incomplete, the Chinese Ministry of Foreign Affairs—on April 7—said that Ai was arrested under investigation for alleged economic crimes.

On April 14, the Beijing government-backed Wen Wei Po newspaper quoted unnamed sources as saying Ai is suspected of evading large amounts of tax and destroying papers that might have been used as evidence against him.

“He has made the government unhappy by speaking up for ordinary people,” Ai’s sister Gao Ge told The Associated Press. “Now the government wants to get him back.”

International governments, human rights groups and art institutions have called for Ai’s release. Chinese officials—as of mid-April - have yet to notify Ai’s family of his whereabouts.

Ai is known for aiding in the design of the bird’s-nest stadium for the 2008 Olympics. His works have been exhibited in nearly 20 nations including the Haus der Kunst in Munich, the Mori Art Museum in Tokyo, and the Tate Modern Gallery in London. In February 2011, a sculpture comprised of 220 pounds of ceramic sunflower seeds sold for $559,394 at Sotheby’s in London.