Department director general Witthaya Suriyawong said Surakrit Chaimongkon, who was suspected of killing Sutin Tharatin, died on Thursday of illness.
Bangkok Remand Prison warder Ayuth Sinthanapun said that Surakrit was sent from prison to a prison hospital at 3.30pm suffering from asthma and breathing difficulties. He died at the hospital at 6pm.
Witthaya reiterated that Surakrit had not died from an assault in detention but from illness. He was known to suffer from diabetes and asthma.
However an autopsy would be carried out as his death occurred while he was in detention.