Police said on Thursday they had questioned eight more witnesses in the investigation into the death of TV actress Nida "Tangmo" Patcharaveerapong, who is believed to have drowned in the Chao Phraya River last Thursday after falling overboard.
But police have yet to pinpoint whose negligence caused the actress to fall to her death, said Pol Lt Gen Jirapat Phumjit, commissioner of Provincial Police Region 1.
"No theories have been ruled out so far, whether it was an accident or if there was something else behind the incident … we aim to make [the investigation] as thorough as possible with the help of forensic evidence," he said.
Among the witnesses grilled on Thursday was Surattanavee "Bow TK" Suviporn, a friend of both Tangmo and her manager, Pol Lt Gen Jirapat said.
In total 29 witnesses including four of the five people who accompanied Tangmo on the boat have now been questioned, he said.
Only two of the people on the speedboat have been charged so far, he added.
Tangmo's five companions were Idsarin "Gatick" Juthasuksawat, her manager; Wisapat "Sand" Manomairat; Nitas "Job" Kiratisoothisathorn; Tanupat "Por" Lerttaweewit, the boat owner; and Phaiboon "Robert" Trikanjananun.
Mr Tanupat and Mr Phaiboon have been charged with operating an unlicensed vessel and negligence leading to death. According to a preliminary investigation, Mr Phaiboon was operating the speedboat when Tangmo fell overboard, Pol Lt Gen Jirapat said.
An analysis of the GPS records is leading investigators to believe the actress fell overboard between 10.29pm and 10.34pm, he said, adding they are now looking into forensic evidence collected from the boat.
The boat was not at the scene when police arrived last Thursday night but Pol Lt Gen Jirapat said no evidence is believed to be missing.
The vessel was seized from NBC Boat Club in Muang district of Nonthaburi the following morning.
Pol Col Chaturon Anurakbandit, superintendent of Nonthaburi's Muang district police station, dismissed rumours that Mr Tanupat recently donated 3 million baht to the station.
"We haven't received the amount claimed and in any case, I insist that everything is being done by the book here."
Investigators on Thursday requested as part of the probe information about the calls made by the five people on the boat from the Technology Crime Suppression Division, who had already collected it, said a source.
Prime Minister Prayut Chan-o-cha has urged police to speed up the investigation.