Sutin Tharatin, a protest leader of the anti-government People's Army and Energy Reform Network, was shot dead in January while he was campaigning against the Feb 2 elections.
Police on July 8 arrested Surakrit Chaimongkol who died in Bangkok Remand Prison on Thursday. Initial reports said he had suffered an asthma attack.
There were no signs of external injuries on Surakrit, Pol Maj Gen Dr Pornchai Sutheerakhun, chief of the Forensic Medicine Institute, told Thai Rath on Saturday.
Gastrointestinal bleeding could be the result of ulcers or painkillers, Dr Pornchai said.
However, Surakrit's mother believes her son may have died from internal injuries suffered in a beating. Aree Chaimongkol said that when she last visited her son on Aug 21, he told her he had been beaten.