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.
Police sent the late actress's mobile phone for examination, which should finish this week
Police have already charged the speedboat owner and the man who was driving the boat when Nida fell to her death.
The actress fell into the river around 10.40pm on Thursday. Her body was found about 1pm on Saturday.
Her companions said she went to the back of the boat to relieve herself and fell into the river.
