"It is not a petrochemical," said Eriel Deranger, spokeswoman for the Athabasca Chipewyan First Nation, which hired the lab that conducted the tests.
Those scientists say the algae bloom was probably the result of high water from record-breaking rainfall combined with unusually warm temperatures.
The Athabasca flows through Alberta's oilsands. Energy companies were asked if any of their facilities had released anything, but none had.
An Athabasca Chipewyan band member first noticed the sheen the night of July 5 while boating between Fort McMurray and Fort Chipewyan.
The town's water intake was closed on Saturday. By Monday, the sheen had been spotted at Fort Chipewyan and the slick was present over more than 100 kilometres of river.
Residents began to spot dead fish floating in the river and on Lake Athabasca.
Samples taken by band members and analyzed in the lab suggest the fish probably suffocated as the out-of-control algae growth used up all the available oxygen.
Deranger said the sheen is still visible in some stretches of the river, but plans were to reopen Fort Chipewyan's water intake Thursday.
Algae blooms occur periodically on the Athabasca — but not like this, Deranger said.
"We definitely have problems with algae blooms, but not this expansive."