A police officer was found stabbed to death, his body mutilated, in Kalasin early on Monday.
Police later arrested a man aged about 40. Investigators said the man's wife had an affair with the victim.
Police, hospital staff and a rescue unit rushed to tambon Kamalasai about 1.30am after the body's discovery was reported.
In a wooden house under construction, the body of a man, wearing a black T-shirt and yellow-black shorts, was found lying face up in a pool of blood. He had been stabbed at least 20 times - on his torso, buttocks and thighs. His genitals were cut off.
The man was later identified as Pol Lt Col Jamras Dolchua, 57, of Rong Kham police station.
Piles of leaves had been set on fire around the house. A fire was also set at the rear of a white Toyota Fortuner parked there. The fires had been put out by neighbours.
A son of Pol Lt Col Jamras told the police that his father usually stayed overnight at a house in Kalasin town. He occasionally spent the night at the wooden house in Kamalasai district.
His father formerly had an affair with a woman who told him she was a divorcee. He later stopped their relationship after learning that the woman was not divorced.
The son believed the woman wanted to return to his father, upsetting her husband. This could be a motive for murder, he said.
