He was a policeman who scored high on his academy exams. He was also a contestant on the local version of the TV game show "Who Wants to be a Millionaire?"

By all accounts, Tsui Po-ko was an above-average citizen. But then he shocked Hong Kong by gunning down a policeman and seriously wounding another in a pedestrian underpass in one of the city's most popular tourist districts.

Tsui, better known by his media nicknames "rogue policeman" and "devil cop," has been the most talked-about person in Hong Kong since the unexplained shooting that also claimed his life about a week ago.

News of the gunfight in the blood-spattered underpass and the investigation have made front-page headlines daily for the past week. It has sparked intense speculation about whether gangsters and other shadowy figures have infiltrated the police. The talk show chatter has also focused on whether troubled officers can be weeded out of the police force.

Tsui, 35, was a tall, lanky man with a square jaw, a crew cut as short as peach fuzz and sweeping eyebrows that looked like the long wings of a flying bat. He was a fitness fanatic, a runner with a sinewy body like a lightweight boxer.

The policeman _ who had a wife and 6-year-old daughter _ was off duty, dressed in khaki pants and a knit hat, when the shooting happened shortly after midnight March 17 in the Tsim Sha Tsui district _ a labyrinth of touristy shops and hotels.

Tsui (pronounced CHOY) ambushed the patrolling officers as they walked down the stairs into the underpass, Lee Ka-chiu, assistant police commissioner, told reporters. Investigators believe Tsui was acting alone, and his attack was meticulously planned, Lee said.

His goal was to steal guns and bullets from the two policemen, said Lee, adding that investigators were still trying to understand what motivated Tsui.

"In my 29 years of service, this is the first case which involves a suspect like this," he said.

One of the details that whipped up interest was that Tsui's gun was stolen from a policeman who was shot dead while answering a bogus noise complaint in March 2001. The same gun was apparently used in the fatal shooting of a security guard during a bank robbery in December that year, Lee said.

If Tsui were alive, he could be prosecuted for the 2001 killings, Lee said.

Early speculation was that Tsui was mixed up with the shadowy gangs called triads. Hong Kong's film industry is famous for churning out ultra-violent gangster movies about the mob infiltrating the police force.

But Lee insisted that Tsui wasn't involved with triads.

David Chan, a senior lecturer of criminology at City University of Hong Kong, said that a few police may be linked to triads, but he doubted that gangsters were entrenched in the force. He said the police are well paid compared to others in the region.

"I'm inclined to think the Hong Kong police force is a highly professional team," he said.

Local media quickly began digging into Tsui's past and found that he was an ambitious and active man. A photograph of him was printed in the weekly Eastweek magazine, showing him marching in a recent pro-democracy protest holding a gold-framed sign saying "Democracy."

Tsui and his wife also appeared on the TV show "Who Wants to be a Millionaire?" in 2001. They won HK$60,000 (US$7,692; euro6,380) on the program, which is to be rebroadcast this weekend. A newspaper photo showed him giving the thumbs up sign on the program.

Dennis Wong, head of the criminology department at City University, said Tsui appeared to be driven by "aggregated frustration." The professor noted that Tsui was passed over for promotions several times over the past five years because he was reportedly deemed too aloof and not a team player.

"Tsui had all sorts of abilities. He was 6 feet (1.83 meters) tall. He was a good shooter. He was very muscular. He could run fast. He should have been promoted, but he wasn't because of the personality defects," he said.

Wong said that when some people can't achieve socially approved goals _ like money, status and power _ by legitimate means, they resort to illegitimate methods.

"This happened in Tsui's case. It's very clear," he said.

But Wong said it would have been difficult for his managers to spot Tsui's dark side because he was a highly rational criminal _ a master at avoiding detection.

James To, chairman of the legislature's security panel, which deals with police affairs, agreed that there wasn't much the police could have done to predict Tsui's behavior. But the lawmaker said the shooting will likely have far-reaching effects on the police force.

"After this case, maybe we'll know the profiles better," he said.