A Chinese professor who wrote a critique of President Xi Jinping’s handling of the coronavirus crisis was placed under house arrest for days, barred from social media and is now cut off from the internet, The Guardian reported.

The newspaper quoted a friend of Xu Zhangrun, who said the professor was confined to his home after his return from a trip to his hometown in Anhui Province under a pretext of being quarantined. “He was in fact under de facto house arrest and his movements were restricted,” said the friend, who spoke to The Guardian on the condition of anonymity. Those restrictions have reportedly been lifted, but the friend said Xu’s internet connection has been cut off since Friday. His account on WeChat, a popular Chinese social messaging app, has reportedly been suspended and his name scrubbed from Weibo, another social media platform. Friends said they are concerned they have not been able to reach him, except through text message in one case.

The Guardian was not able to reach Xu, a professor of law at Tsinghua University, directly. The newspaper’s calls to China’s Ministry of Public Security were not answered.