Police have not ruled out motives other than a hit by underground gambling interests.

Prayuth Natetrayont was shot dead in his car at an intersection in Dusit district on Sunday

night. Police said two men on a motorcycle fired shots. They added the manner of the shooting suggested the killers were experts.

Prayuth had close ties with vocal anti-gambling campaigner Santhana Prayoonrat. Santhana ran for the Senate in the last election. He is also a former deputy commander of the police special branch.

He has led a years-long campaign against illegal casinos in the Tao Poon area of Bang Sue district - considered notorious for its rampant underground gambling.

Police said the shooting could have been related to several activities of Santhana. Metropolitan Police division 1 commander Maj-General Manit Wongsomboon, who is heading the case, said Santhana operated a racehorse stable and had an interest in a used-car dealership.

He said that Santhana had evicted tenants from the location the dealership was built at. In addition, rivals in the racing world could be involved.

But Santhana was convinced his campaign to rid Tao Poon of illegal gambling was behind the shooting of his aide and vowed to respond to any violence directed at him.

He alleged the murder was ordered by a group of influential people with illicit business networks.

"They want to stop me from exposing their illegal activities and gambling operations," he added.

Santhana claimed he had entered a Tao Poon den in disguise just one day before Prayuth was

shot. He claimed many casinos operated freely in the Tao Poon community.

"If they are allowed to operate in Tao Poon without disruption, that means there are dens open freely elsewhere," he added.

Lt-General Wiroj Jantharangsee of the Metropolitan Police earlier admitted Tao Poon casinos operated in spite of extensive police crackdowns.

Tao Poon police chief Colonel Pongsak Sumitnant said officers conducted 21 raids on suspected casinos this year, sometimes turning up nothing.

Pongsak claimed a Santhana tip-off on a busy gambling den led to a raid on an empty house. He dismissed news reports that gambling was rampant in his jurisdiction because of corrupt or incompetent officers.