Investigations began immediately after the discovery of three unconscious men on the floor of a pedestrian subway at Canton Road and Austin Road.

Officers, joined by a canine unit, combed the area with guns drawn, fearing other armed men.

But by late afternoon, the evidence- gathering work centered on the subway at the southeast corner of the intersection.

Police officers, mainly from three units - uniform beat, police technical, and crime officers - searched for clues as to how two uniformed policemen became involved in a bloody clash with an off-duty officer shortly after am.

Both the off-duty officer, 35-year- old Tsui Po-ko, and one of the uniformed men, 33-year-old Tsang Kwok- hang, were pronounced dead soon after they were rushed to Queen Elizabeth Hospital Friday.

The other officer, 28-year-old Sin Kar-keung, remained in critical condition.

Tsang's family, including his wife and mother, visited the scene to mourn and burn incense.

His mother, amid tears, said she missed her son.

The victim's wife tied a black T-shirt on the railing of the subway as she cried and called out his name.

"Kwok-hang let's go home," she said after retrieving the T-shirt.

Hidden under a tent, police authorities worked all Friday to gather fingerprints and other clues.

"Generally speaking, they were collecting evidence," a police spokesman said.

"We cannot disclose the details of what they used."

But from the street level, authorities were seen using a fluorescent yellow liquid and ultraviolet light on the subway walls, a method used to highlight fingerprints and blood stains.

The police had collected the radios, hats and revolvers that were left behind when the three police officers were taken to hospital, a grim reminder of the lives lost.

Shortly after 5.15pm, scores of officers from the investigation team emerged from the subway with their equipment, but the area remained cordoned off and the yellowed walls unwashed until later in the evening.

The incident was a rude awakening for residents in the sleepy neighborhood.

The shots startled many, including one elderly woman who lives in a flat that overlooks the subway.

She said she was trying to sleep when she heard "bang, bang, bang," and thought there had been a car accident.

But when she saw so many people outside, she thought a movie shoot was the source of the noise.