Following the murder of policeman Leung Shing-yan and the theft of his revolver five years ago about 3,000 people - 2,000 of whom were police officers - were questioned.
But the suspected rogue cop at the center of last week's shootout, Tsui Po- ko, flew under the radar until Leung's gun was found in his hand last Friday.
"The revolver used by the suspect in the Tsim Tsa Tsui shooting case has been confirmed to be the gun stolen from the late police constable Leung who was robbed and murdered at Shek Wai Kok," assistant commissioner of Police (crime), John Lee said Monday.
The same gun was also used in a violent bank robbery in December 2001 which resulted in the death of a security guard.
Tsui, 35, is suspected of ambushing two patroling officers in the pedestrian subway at Canton and Austin roads.
Investigators believe the suspect, an officer from the North Lantau division, initiated the shootout to steal the firearms and ammunition of the patroling officers, Lee said.
He stressed there was a need for more tests to be performed on the revolver as it was rusty when found at the scene.
"At this stage, we cannot draw a definite conclusion about its connection, ballistically, to the two previous cases," Lee said.
But Lee pointed out that there might be a link between the total number of shots fired by the gun and the total number of bullets stolen from Leung five years ago.
He said Leung was shot six times. Counting the three shots during the bank robbery and another three from last Friday's subway shootout, it makes a total of 12 shots - the number of bullets Leung had on the night he was murdered.
The subway confrontation resulted in the deaths of Tsui and a uniformed officer, Tsang Kwok-hang, 33.
The second patrol officer, Sin Kar- keung, 28, was hit in the leg and face but his condition is now stable.
Lee said Tsui did not have a history of mental illness but that the police are trying to find out what were his motives and state of mind at the time of the shootout.
He said the Organized Crime and Triad Department was looking into different angles, including speculation that Tsui was involved with an underground gambling syndicate.
Nine months after Leung's death, the gun reappeared in a bank robbery. Security guard Zafar Iqbal Khan, 31, tried to stop a masked gunman from taking about HK$500,000 from the Tsuen Wan branch of Hang Seng Bank. But Khan was shot three times and was declared dead upon arriving in hospital.
Witnesses to the robbery described the suspect's appearance to authorities at the time but no arrests were ever made.