Friday’s opening festivities for the Shenzhen-Hong Kong Biennale of Urbanism/Architecture were marred by the arrest of Chinese-French artist Hu Jiamin and his wife, Marine Brossard, according to news reports. The artist had painted a mural in tribute to Liu Xiaobo, the Chinese dissident and Nobel Peace Prize winner who died a political prisoner in state custody earlier this year.

Police covered the mural—prominently located at the exhibition’s entrance—with a banner featuring information about the biennial, and the artist and Brossard have not been heard from since their arrest, according to Art Asia Pacific.

Titled Time Discrepancy (2017), Hu’s painting featured an empty blue chair, a reference to the award ceremony for Liu’s 2010 Nobel Peace Prize ceremony; imprisoned by the Chinese government, Liu could not travel to Oslo for the occasion, but an empty chair stood on the stage in his honor.

Terminally ill with liver cancer, Liu was released on medical parole in June and died the following month under strict guard in the hospital. He had been arrested in 2008 and sentenced in December 2009 to 11 years in jail for “inciting subversion of state power.”

The South China Morning Post spoke to a biennial staffer who confirmed that the police investigation was due to “political problems” with the artwork, which also featured iron bars obscuring the view of a traditional Chinese landscape and surveillance cameras.

Hu Jiamin’s Time Discrepancy (2017), detail, at the Shenzhen-Hong Kong Biennale of Urbanism/Architecture, with the artist’s wife, Marine Brossard. Photo courtesy of Free Liu Xiaobo via Twitter.

“I have been very upset since I heard about Liu’s passing away,” Hu told the Post, adding that “I’m not a radical person, nor am I an activist… I painted the chair to express my personal commemoration and grief towards Mr. Liu, but it’s not a manifesto to the public.”

Chinese writer Ye Du, a friend of Liu’s widow Liu Xia, told Radio Free Asia that he believes the couple is being held due to the artwork’s political content, but that he could not be sure. “I won’t say any more right now,” he said, “because my freedom has also been restricted, and we don’t actually know their situation right now.”

The biennial was organized by art critic and curator Hou Hanru, who co-curated the exhibition with Liu Xiaodu and Meng Yan. Hou declined to comment for this story when contacted by artnet News; attempts to reach Liu and Meng were unsuccessful.

Dedicated to urbanism and now in its seventh edition, the biennial will be on view across multiple sites in Shenzhen and neighboring Hong Kong through March 17, 2018.