Prime Minister Prayut Chan-o-cha has vowed he will "spare no one" if Monday night's shooting in Bangkok involved illegal police activity in connection with underground gambling.
Gen Prayut was reacting to the fatal shooting of Pol Maj Watthanaset Samniangprasert, an interrogation inspector at Samae Dam police station in Bang Bon district, and three other people, one of them a woman, at a gambling den in Yannawa district.
Three of those killed were shot by a losing gambler, who was in turn killed by another man now being hunted by police. He is believed to have fled through the back door of the building.
The premier said any police officers who abused their authority in connection with illegal casinos would be punished.
"Yes, the prime minister has total authority to oversee the police. The bad eggs will face punishment. No one will be spared," Gen Prayut said.
His comment followed confirmation by Bangkok police chief Pol Lt Gen Pakapong Pongpetra that a man of Chinese appearance had shot dead Pol Maj Watthanaset and two other people.
The shooter was then himself shot dead by another man, who fled the premises, Pol Lt Gen Pakapong said.
The den was on the ground floor of a four-storey commercial building surrounded by security cameras about 300 metres off Rama III Road in Soi 66.
The premises had strong, opaque doors and access appeared heavily restricted. There were chairs and tables, plus monitor screens on the walls.
A police source said on Tuesday that the building in question had been rented out several times in recent years. Most recently, it was leased to a man named Tor, who opened what became a popular casino.
There were about 60 gamblers in the premises at the time of the shooting.
Neighbours said the den often happened to be closed when the authorities raided it, only to reopen afterwards.
They did not, however, hear Monday night's gunfire because the room had noise insulation.
Pol Lt Gen Pakapong said on Tuesday Pol Maj Watthanaset was not in the room on official duty and had not been assigned to bust the den.
The commissioner said at this stage police suspected a personal feud had motivated the shooter to open fire on Pol Maj Watthanaset. The two men might have been trying to clear the air about something but could not settle the issue, he speculated.
According to Pol Lt Gen Pakapong, Pol Maj Watthanaset's superior said the slain officer had a penchant for gambling.