His brief encounter with Wilasa Janbanchorn, whose doctor husband is suspected of killing as many as four people, took place on Saturday at a police station in Phetchaburi province.
"I asked her, 'Where did you take my son and daughter-in-law?,'" he told reporters at the Thai Mai Ruak station in Tha Yang district, where Mrs Wilasa is being held. "I don't know where the doctor took him," he quoted Mrs Wilasa as saying.
"But she told me that she was with her husband when the couple were taken to his pineapple orchard," said Mr Sawang. "I still believe that she definitely knows." Mr Sawang had gone to the police station at 9.20 Saturday morning after learning that Mrs Wilasa had been brought there after her arrest in Bangkok the day before.
Mrs Wilasa rejected his request to see her but was persuaded to change her mind by Pol Col Pichai Pokpong, the station chief.
Mr Sawang said his only intention now was to recover the bodies and hold a religious ceremony for his son Samart and his wife Orasa Kersap.
The couple were last seen at a pineapple orchard owned by Supat Laohawattana in June 2009. Mr Sawang says he accepts that they are probably dead.
Pol Col Pichai took Mrs Wilasa to the provincial court, which denied her bail on grounds that she might tamper with evidence of the investigations and flee if she were released.
Mrs Wilasa, one of four wives of Dr Supat, was arrested on Friday in Taling Chan district in Bangkok by officers from Provincial Police Region 7 and taken to their office in Nakhon Pathom. She was sent to Phetchaburi later that night.
Phetchaburi provincial police questioned Mrs Wilasa for eight hours until 3am on Saturday before transferring her to the Thai Mai Ruak station.
Mrs Wilasa faces the same charges as her husband: theft, possession of stolen items and illegally restraining people against their will.
Her husband, a former medical doctor at the Police General Hospital in Bangkok, is suspected of murdering the missing couple, as well as two Myanmar nationals.
Police said Mrs Wilasa told them on Friday that she was with Pol Col Supat when he took the couple to his pineapple orchard in Tha Yang on June 19, 2009.
But she said she did not know where they were taken after that, and that all she heard was a sound like shots from a gun with a silencer.
Three skeletons were dug up at the doctor's orchard last month after he became a suspect in the disappearances, but the results of DNA tests did not match the missing couple.
Evidence showed that at least two of the bodies had been shot to death.
Relatives of Mr Samart and Mrs Orasa still suspect that their bodies are buried somewhere in the orchard.
Three skeletons were dug up from the orchard on Sept 20 but DNA tests showed none matched the missing couple. The identities of the skeletons have still not been established.
The Provincial Police Region 7 on Friday sent DNA samples taken from the mother and son of a Myanmar worker called Ita to the Police General Hospital to see if they matched any of the skeletons found at Pol Col Supat's orchard, according to a police source.
Pol Col Supat remains in custody in Phetchaburi but police have not yet filed a murder charge against him.