Police plan to use lie detectors when they question people who were on the speedboat with TV actress Nida "Tangmo" Patcharaveerapong when she fell into the Chao Phraya River and drowned last week.
The national police chief said on Monday that a lie detector would help the investigation.
"Every answer from Nida's friends must be backed by reasons... Police will see who they phoned after the incident... and if there was any physical assault. Police will make it clear if there was any dispute or life insurance involved," he said.
Tangmo's mother, Panida Siriyutthayothin, has said she did not believe her daughter's death was an accident and she suspected foul play.
Police on Monday discussed progress in the investigation into the death of the 37-year-old woman, who fell from a speedboat into the Chao Phraya River in Nonthaburi last week.
