Police last night stepped up investigation into the criminal intimidation against legislator Anson Chan Fang On-sang after several media organisations received identical threatening notes. The writer claimed responsibility for the letter sent to Mrs Chan. Heavily armed police later went to a flat in Wong Tai Sin but ruled out any involvement by the occupants. In a type-written letter received by the South China Morning Post, the sender claimed to be the 'Mass Murderer' who sent a razor blade and a threatening note to Mrs Chan last week asking her to commit suicide. He said he knew the whereabouts and living habits of Mrs Chan. 'Although I have a gun, I never thought of killing her, because killing her will only make my hands dirty,' the writer said. The Post reported the matter to the police. Mrs Chan's office also notified police after being contacted by the media. The writer gave an address in a public housing estate in Wong Tai Sin. A team of heavily armed police officers wearing body armour went to the address yesterday afternoon. After initially refusing to open the door, a woman, with her son and daughter, yielded peacefully. Neighbours alleged that the man, believed to be in his late 30s, had a record of causing nuisance in the estate and had openly claimed to have mental problems. After a two-hour investigation, a police spokesman said the flat's residents were not the senders of the note. No one was arrested. The Hong Kong Island crime unit is handling the investigation. The threatening note to Mrs Chan just three days after being sworn in as a legislator caused a stir in political circles. Commissioner of Police Tang King-shing said last week that he was 'highly concerned' about the case, with politicians from all sides urging investigations.