Chinese police have widened the investigation of avant-garde pop artist Ai Weiwei, calling friends in for questioning. As part of a major crackdown on dissidents, the police took Weiwei into custody Sunday and he has not been seen or heard from since.

It is not the first time the designer of the Bird's Nest Olympic stadium and digital activist has been in trouble with the government. In 2009, after protesting the deaths of thousands of children in the Chengdu earthquake, Weiwei was taken into custody and beaten. He later suffered a brain hemorrhage as a result.

Weiwei’s work is often likened to Andy Warhol’s for its pop deconstructions of familiar images., the L.A. Times wrote in March. It also shares a similar thread of mocking iconic imagery. One of his most famous series of works consists of photographs of Weiwei flipping off monuments, including Tiananmen Square and the White House. His large installations have received accolades from around the world, but in the past two years, he has become better known for his digital work on Twitter as a pro-democracy microblogger.

Read more about the fearless artwork of Weiwei at Slate. Here are some of Weiwei’s well-known artworks, courtesy of the artist and Mary Boone Gallery in New York.

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