HONG KONG (AP) — Four members of a Hong Kong university student union were arrested Wednesday on accusations of advocating terrorism when they paid tribute to an attacker who stabbed a police officer and then killed himself, police said.

Those arrested included the president and council chairman of the University of Hong Kong’s student union, who were among more than 30 students who attended a meeting last month in which they passed a motion to mourn the “sacrifice” of the attacker, local media reported.

The arrests are the latest use of a strict national security law that Beijing imposed on semiautonomous Hong Kong last year after months of antigovernment protests. The protests which roiled the former British colony for months in 2019 often led to clashes between demonstrators and police.

Authorities have said the the man who on July 1 stabbed the officer in the shoulder before killing himself had material in his home decrying the national security law as well as notes where he declared his hatred for the police.

Police previously cautioned people against mourning for the attacker, saying that mourning him is “no different from supporting terrorism.”

The student union withdrew its motion mourning the attacker after facing criticism from the government and the university and some union members quit. The university responded by severing ties with the union.

More than 100 pro-democracy activists have been arrested under the national security law, which outlaws subversion, secession, terrorism and foreign collusion to interfere in the city’s affairs.

Critics say the law has been used to stifle dissent and restrict freedoms Hong Kong was promised it could maintain for 50 years following its 1997 handover to China.