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.
National police chief Pol Gen Suwat Jangyodsuk said on Monday that a lie detector was a tool to facilitate 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," the national police chief said.
Tangmo's mother, Panida Siriyutthayothin, has said she did not believe her daughter's death was an accident and she suspected foul play.
Pol Gen Suwat went to the Cyber Crime Investigation Bureau on Monday to discuss 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 Thursday night.
Pol Col Chaturon Aksornbundit, chief of Muang Nonthaburi police, said lie detectors would be used when questioning Tanupat "Por" Lerttaweewit, the owner of the speedboat, Phaiboon "Bert" Trikanjananun, Nitas "Job" Kiratisoothisathorn, Tangmo's manager Idsarin "Gatick" Juthasuksawat and Wisapat "Sand" Manomairat.
Invesrigators hoped this would provide a clearer picture of what happened, he said.
After initial questioning, the Muang Nonthaburi police chief handed his report to Pol Gen Suwat on Monday.
The five people's initial accounts went along with each other, but police were looking into all angles that interested society, Pol Col Chaturon said.
Pol Col Siriwat Deepor, deputy commander of the Cyber Crime Investigation Bureau, said Muang Nonthaburi police sent the late actress's mobile phone to the bureau for examination, which should be concluded this week
Police have already charged Mr Tanupat, the speedboat owner, and Mr Phaiboon, who was driving the boat when Nida fell to her death on Thursday.
They are charged with operating an unlicensed vessel and with the more serious charge of negligence leading to the death of a person.
The second charge carries a maximum penalty of a 200,000 baht fine and/or 10 years imprisonment. The two men were released on bail on Sunday.
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 toppled into the river.