A 35-year-old man has been detained for interrogation in the murder of his father and younger sister, whose bodies were discovered in locked steel boxes dumped in a pond in Sakhon Nakhon.

Residents of Ban Sang Khui in Phang Khon district told police on Monday that two large steel boxes they had seen next to the pond were suddenly missing.

Police and rescue workers found the two boxes in the pond about 1am on Tuesday. They were raised to the surface and dragged ashore. The boxes were brand-new and held closed by steel chains. Upon opening,  each was found to contain one body.

The bodies were later identified as Prayat Detphummee, 66, and his daughter Piangpen, 33.

Police detained a 35-year-old man at the scene who was shouting and talking senselessly.  He was identified only as Waroot. Police learned that he was Prayat's son and Piangpen's elder brother.

The Thai News Agency reported that the family came from Sawang Daen Din district. A metal worker there told police Mr Waroot had bought two large steel boxes from him on Nov 25 last year for 12,000 baht.

He said Mr Waroot claimed he planned to the use the two cases to store fishing equipment.

Police said relatives had been told the family were taking a trip to Tak province.