Police are not convinced by statements given by five people on the speedboat with TV actress Nida "Tangmo" Patcharaveerapong when she fell overboard to her death, Pol Lt Gen Jirapat Phumjit, head of Provincial Police Region 1, said on Tuesday.
"The cause of death is found [at this point] to be drowning. To determine what actually caused her to fall into the river, whether it was herself or someone else, will take time.
"As for the statements given by the five people in the boat, police aren't totally convinced," he said.
The actress reportedly went to the back of the boat to relieve herself when she fell in the water on Thursday night. Her body was found on Saturday afternoon.
The five other people on the boat were: Idsarin "Gatick" Juthasuksawat, Tangmo's manager; Wisapat "Sand" Manomairat; Nitas "Job" Kiratisoothisathorn; Tanupat "Por" Lerttaweewit; and Phaiboon "Robert" Trikanjananun.
Mr Tanupat, the boat owner, and Mr Phaiboon, the driver, were charged with operating an unlicensed vessel and negligence leading to death.
On Tuesday, they were asked to undergo a physical examination.
In particular, Mr Nitas who was found during an interview with the media on Monday to have some scratches and bruises, said Pol Lt Gen Jirapat.
They are required to submit for inspection clothes they wore on the boat trip, he said.
Only Mr Nitas was found to have scratch marks and bruises, according to a doctor who spoke at a police press conference on Tuesday.
The doctor, however, determined the scratches were not inflicted by a person's nails or caused by a struggle, adding they were about five days to a week old.
As part of the ongoing investigation, the police have begun a five-day reenactment of the tragedy, in which they plan to take the five one by one on a similar speedboat to try and determine what led up to the actress falling overboard.
The reenactment is to find any inconsistencies in the statements given by the five people, according to Pol Lt Gen Jirapat.
The reeneactment will take five nights as the investigators want to do it at about the same time as the tragedy with only one witness in the boat on each night.
Statements given by four witnesses fishing in the river will be taken into account, Pol Lt Gen Jirapat said.