Police believe the mastermind is still at large for the abduction and torture of a Japanese businessman who was rescued last Friday.
Pol Lt Gen Sanit Mahathavorn, commissioner of the Metropolitan Police Bureau, said on Monday some of the three suspects already in custody had allegedly been hired to commit the crime but none of the others could afford to do so.
Therefore, it was highly likely there was a mastermind who could pay for it, the Bangkok police chief said.
He was referring to the rescue of Tsunemi Watanabe, 56, from Room No.719 at the Grand Hi-tech Tower apartment building on Ekamai Soi 23 in Watthana district last Friday.
The owner of a medical equipment distribution business was seen tied to a bed and sustained serious injuries to his legs and bruises on his body.
At the apartment building, police arrested Masato Kobari, 45, Reo Tsuruzoe, 41, and Kiyoto Miyata, 57. Pol Lt Gen Sanit said Mr Reo was allegedly the gang leader but the Thai police could not find any transnational criminal record of the gang.
The victim was not ready for lengthy interrogation but initial questioning found that the man might be abducted for ransom, the Bangkok police chief said. He also said that police would examine the victim's assets and businesses.
The Southern Bangkok Criminal Court on Monday approved the first detention period of 12 days for the three Japanese suspects who were charged with detention and physical assault.
Police told the court that from June 5 the suspects detained Mr Watanabe, tied him to a bed with electric wires, blindfolded him with a black cloth and used a baseball bat to break his right leg. Police initially attributed the crime to a construction conflict. The three suspects denied all charges.