As evidenced by the Pulrose Skirmish, when the police carry guns, they shout a lot. It is part of their training.
So the occupiers of the building find someone sneaking into their back door, wearing balaclavas and carry guns. They think it is another Triad gang mounting a raid, so they smack them over the head with a baseball bat - or shoot them.
Anyway, if the police have arrested someone, why have they not announced it? They do that frequently.