'The units were ransacked. The victims' personal belongings, such as mobile phones, were missing," he said. "We do not rule out the possibility that the cases are committed by the same person(s)."

A non-governmental organization which provides assistance to sex workers, Zi Teng, expressed shock and fear over the deaths of the prostitutes.

"We feel sad about that. We hope their deaths will not go unnoticed," said the group's spokeswoman Elaine Lam.

Officers from the Identification Bureau are examining the crime scene of the murder of the fourth victim in a recent spate of prostitute murders in North Point yesterday. Two more prostitutes have been murdered, bringing the total over the past three days to four. Edmond Tang

Lam said prostitutes in Hong Kong were discriminated against, and could not get assistance from police.

City University of Hong Kong's applied social studies associate professor Wong Sing-wing believed that the case was committed by one person for money, but not out of hatred against the prostitutes.

"The sex organs of the women were not chopped off. According to previous cases in foreign countries, prostitute-killers who hated prostitutes would chop off their victims' sex organs. That did not happen in these cases. The victims' homes were searched and their belongings were taken away. It is possible that the killer needed money," he said.

Wong also believed that the killer was well aware of the geographical characteristics of the crime scenes. The victims had published their photos, addresses and maps on the Internet.

The killer has intentionally avoided busy districts, such as Mong Kok and Tsuen Wan, Wong said.

"Probably, the killer has visited the locations before committing the crimes," he said.

Police investigation will be difficult because of the lack of closed circuit television inside the buildings, Wong added.

Legislative Council security panel chairman Lau Kong-wah urged the police to strengthen resources in the investigation of the cases.