A group of blackmailers led by a psychiatric patient threatened to fire a shot near the head office of Sun Hung Kai Properties unless its chairman, Walter Kwok Ping-sheung, handed over $15 million, a court heard yesterday. Mental patient Yip Chi-wai and Yau Ming-fong, both 36, yesterday pleaded guilty to a joint charge of blackmail. The extortionists posed as Xinhua staff in a telephone call to the tycoon's private secretary, Chan Mei-ka, on August 10. Ms Chan thought the callers were seeking donations for mainland flood victims. Mr Kwok was busy and Ms Chan passed the message to the company's public relations manager, Lau Mei-mui, two days later. Ms Lau returned the call, only to be told of the $15 million demand, the District Court heard. A policewoman then posed as Mr Kwok's personal assistant and took over all contact with the blackmailers. Prosecutor Steve Chui Fook-leung said the policewoman received a telephone call from Yip the same day. Identifying himself as Wong, he repeated the demand for $15 million. Otherwise 'they would show force by sending someone to discharge a shot by a Black Star pistol in the vicinity of their company, and people in their company may get hurt', Mr Chui said. 'Once that happened, they would ask for $150 million instead of $15 million.' Yip then gave the policewoman a bank account number in Yau's name and a telephone number for future contact. Further calls the next day were traced to different places, including a restaurant in Wong Tai Sin. Police mounted surveillance at the restaurant later the same day where Yip, Yau and another man turned up. Whenever Yip was observed making a telephone call, the policewoman received a call from 'Wong' at Mr Kwok's office. The demand was negotiated down to $6 million and the policewoman said the money would be deposited. Police moved in and arrested Yip and Yau. Their co-accused, Wong Keung-yiu, 32, did not enter a plea. His counsel, Johnny Chan Jong-herng, said his client was considering terminating his services. Judge Esther Toh Lye-ping adjourned Wong's case to today. She postponed sentencing of Yip and Yau to February 22, pending an updated psychiatric report on Yip.