The university’s students’ union council had earlier passed a motion “appreciating the suspect’s sacrifice” for Hong Kong. It later withdrew the motion and apologised, and all council members resigned.
Siu said on a TVB programme that glorifying what the suspect did could be a violation of the security law’s ban on promoting and inciting terrorism.
If what they did causes hatred among different groups of people in Hong Kong, it could also amount to sedition, he said.
The police commissioner said officers conducted a search at the university campus on Friday to try and learn about the intentions of those involved.
He said others who had mourned the suspect’s death with flowers in Causeway Bay could also be investigated, but that depends on whether they had taken further action such as publishing seditious words.
Meanwhile, Siu said the police plan to step up counter-terrorism drills.
He said not only will law enforcement agencies take part, the MTR Corporation, malls, hotels and even the public will be invited to join, so as to raise awareness.