Police on Friday arrested the wife of Supat Laohawattana, a former Police General Hospital doctor under investigation for the 2009 disappearance of a couple in Phetchaburi.
Wilasa Janbanchorn was arrested at a friend's house in Taling Chan district of Bangkok and taken for interrogation to Police Region 7 headquarters in Nakhon Pathom, said Pol Lt Gen Harnpol Nitwibul, the regional police chief.
Police have charged her with theft and illegal detention in collusion with Pol Col Supat.
Samart Noomjui and Orasa Kerdsap have been missing since 2009. Mrs Wilasa and Pol Col Supat were seen with them on June 12, 2009. Police found two guns in her apartment in Phasicharoen district of Bangkok.
Mrs Wilasa told the police that she was with her husband when he took Mr Samart and Mrs Orasa to his pineapple orchard in Tha Yang district in Phetchaburi province on that day.
But she said she did not know where the couple were taken after that, and all she heard was a sound like shots from a gun with a silencer, according to Pol Lt Gen Harnpol.
Pol Lt Gen Harnpol said investigators were seeking more information from the woman before handing her over to the Tha Mai Ruak police station in Phetchaburi on Saturday.
The Phetchaburi police will seek permission from the provincial court to detain her, he added.
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.
Authorities are also preparing to conduct DNA tests on family members of a Myanmar labourer to see if their DNA matches that of one of the skeletons found on the doctor's property.
Pol Col Supat remains in custody in Phetchaburi but police have not yet filed a murder charge against him.