Chinese authorities have detained Deng Chuanbin, an independent filmmaker and activist also known as Huang Huang, for tweeting a photo alluding to the Tiananmen Square massacre of 1989.

Early on the morning of May 17, just hours after posting the photo, authorities arrived at the filmmaker’s home in the Sichuan province with an arrest warrant. They confiscated his electronics—cell phones, computers, cameras, and memory cards—and escorted him to the Nanxi District Detention Center, where he has been held since, according to ArtAsiaPacific.

Chinese Human Rights Defenders, a grassroots activist group, reported that authorities returned to Deng’s home at least one additional time after the arrest to collect more of his belongings.

The photo of a clear bottle of liquor with a label marked “64” is a coded reference to the month and day of the Tiananmen Square protests: June 4, 1989. The name of the alcohol, bai jiu, sounds like the Mandarin word for “89” (“ba jiu”).

This isn’t the first time that Huang, who has collaborated in the past with dissident artist Ai Weiwei and filmmaker Nanfu Wang, has been detained. In May 2015, he was held for questioning by Chinese officials, who confiscated his passport and prevented him from attending a human rights conference in Geneva. In 2016, Huang appeared in Wang’s Hooligan Sparrow, a documentary about a group of activists fighting a child rape case involving a government official.

A bai jiu bottle alluding to the Tiananmen Square massacre.

Next month marks the 30th anniversary of the Tiananmen Square tragedy, in which the Chinese military invoked martial law and attacked protesters with rifles and tanks, resulting in thousands of civilian deaths. The Chinese government has since gone to great lengths to suppress any mention of the event, particularly in the leadup to the 30th anniversary, now less than two weeks away.

Activists throughout the country have reported experiencing extra surveillance measures, while others, like Deng, have been arrested for publicly referencing the historic protests. In April, four men who created real-life replicas of the “64” bai jiu bottles in 2016 were sentenced to several years in jail (after already spending three years in “pre-trial detention”).

Meanwhile, an inflatable sculpture of the protester who famously stood before a line of tanks in Tiananmen Square, appeared in Taipei, Taiwan, last night. The work, by Taiwanese artist Shake, was inspired by a drawing by dissident Chinese political cartoonist Badiucao.