Police have found a big cut on the thigh of late TV actress Nida "Tangmo" Patcharaveerapong who drowned in the Chao Phraya River last week and a scratch on her boat companion.

Police said an autopsy confirmed that Tangmo, 37, drowned because there was mud in her lungs.

Police found a big cut on her right thigh but had yet to confirm if it came from a boat propeller. There were also small scratches in her left leg and police had yet to identify their cause.

Boat companion Nitas "Job" Kiratisoothisathorn had a scratch on his arm and a bruise on his shoulder. The scratch was bigger than a fingernail scratch and the bruise was caused by an impact with a hard object.

They were five to seven days old, police added.

Police said they should know in three to four days if there were any traces of urine on the rear of the speedboat and on Tangmo's bodysuit and napkin. One of her boat companions said earlier that the actress fell while relieving herself on the rear of the boat.

Police said officers had not been able to conduct an examination on Tangmo's five speedboat companions instantly as they had to wait for their consent. They were considered witnesses, not suspects, police added.

Police were waiting for their blood test results and DNA testing will also be carried out.

Tangmo fell into the river around 10.40pm on Thursday. Her body was found about 1pm on Saturday.