The SIU assigned four investigators after the man jumped into the river and disappeared on Wednesday.
"Police officers went into the river to apprehend him but were unsuccessful," says the SIU news release.
On Friday, Ottawa police said it presumes the man drowned after a two-day search could not locate his body. Police divers continued to search the Rideau River near Riverain Park on Friday afternoon as part of a recovery mission.
A day earlier, police forces in Ottawa and London, Ont., said they were investigating whether the man is Sultan, from Calgary.
Sultan, 23, is wanted on a charge of second-degree murder in connection with the death of 18-year-old Cook, who was shot and killed while trying to track down his lost cellphone on June 14 in London.
Cook's funeral took place on Monday in Brampton, Ont.
Altercation with police cyclists
The man police say drowned also allegedly struck two bike-riding police officers with his car on Wednesday in Ottawa's Lowertown neighbourhood.
Police said he then tried to swim across the Rideau River and ran into trouble in the water, so two officers jumped in the river to try to rescue him. He then slipped under the water, police said.
Last week, London police said they are looking for a second suspect in Cook's death. He is described as a black male with very short hair. He was last seen wearing a black jacket or shirt and a fitted cap.
A third man, who got out of the vehicle just before the London shooting, is being asked to contact police and identify himself.