Update: 12.35pm: It has emerged a man man found dead in a river near Thornlie yesterday had run away from police about a week ago.
It is believed the officers lost sight of the man, who was wanted by police, and lost sight of him in the area. They did not see him go into the river and it is unclear whether he entered the water at that stage.
The Major Crime Squad is investigating the death and internal affairs officers have asked them to ensure they check out the police sighting of the man.
Police are still trying to track the man's movements in the past week to determine when he died.
They have not released the man's name or details of how long they think he was in the water.
The man's body was found by students kayaking on the Canning River yesterday.
His clothed body was found partially submerged near the bridge at Royal Street in Thornlie about 8.30am.
Kelmscott high school students and their teacher found the body floating face down soon after they started kayaking.
Year 11 student Taine Kingi said he and his classmates could see only a head above the water near the river's edge and were horrified when they learnt it was a dead body.
"We were all pretty shocked and traumatised," Taine said. He said the school had offered counselling.
Police cordoned off big sections of the river, surrounding parkland and paths on both sides of the road where Royal Street becomes Spring Road.
Forensic experts spent several hours scouring for evidence before water police divers retrieved the body about 2pm yesterday.