Former national police chief Somyot Poompanmoung reported to the Department of Special Investigation on Thursday for questioning about his "borrowing" 300 million baht from the owner of Victoria's Secret Massage parlour while in office.
Pol Gen Somyot met DSI interrogators at the division handling human trafficking cases in Bangkok.
The DSI summonsed him to provide information on his financial transactions involving Kampol Wirathepsuporn, the fugitive owner of the massage parlour.
Pol Gen Somyot, currently president of the Football Association of Thailand, did not talk to news reporters before going in for questioning.
Pol Lt Col Supat Thamthanarak, DSI director for human trafficking cases, said Pol Gen Somyot would testify as a witness in a human trafficking case relating to Victoria's Secret Massage parlour.
Questions would focus on financial transactions and the 300 million baht Pol Gen Somyot borrowed from Mr Kampol.
After interrogation lasting about two hours, Pol Gen Somyot continued to reject reporters' questions. He said he could not give any detail because the matter is already in the justice system and was being investigated by authorities. He hoped they would guarantee justice.
He did not answer why he had not reported the loan from Mr Kampol to the National Anti-Corruption Commission.
He then left the DSI immediately.
Pol Lt Col Supat said no charge had been brought against Pol Gen Somyot, who was considered a witness.
Pol Gen Somyot also gave some documents to DSI officials, he said.
The retired police chief earlier admitted he had borrowed the money from Mr Kampol, and said they had been friends for two decades.
Mr Kampol and his wife remain at large. Authorities allege the massage parlour had many underage girls who were forced to provide sex services.
The parlour, in Huai Khwang district, Bangkok, was raided on Jan 12. Investigators later uncovered the financial transactions between Pol Gen Somyot, when he was police chief, and Mr Kampol.