A toxic spill is the most likely cause behind the discovery of hundreds of dead fish in the Ottawa River earlier this month, necropsies suggest.

Residents began noticing the dead fish in the water and along the shore of the river near Cumberland in Ottawa's east end.

Within the past week, a wildlife health centre in Saint-Hyacinthe, Que., conducted necropsies on 12 fish carcasses of various species found in the river.

They found no sign of infectious disease, leading to the conclusion the fish were exposed to a toxic agent that was either accidentally or purposely dumped into the water, according to Stéphane Lair, director of the Centre québécois sur la santé des animaux sauvage.

New findings on the Lièvre River Fish Kill have been released by Quebec's MFFP. As we expected, they point toward the cause being a spill, but are unfortunately inconclusive as to what was spilled. <a href="https://t.co/gDhF8C1G4C">pic.twitter.com/gDhF8C1G4C</a> —@ottriverkeeper

Officials believe the dead fish originated in the Du Lièvre River in Quebec, which flows into the Ottawa River at Masson-Angers, just across the from Cumberland. Only a water analysis taken at the time of the incident could have identified the toxic substance, Lair told Radio-Canada.

No more dead fish have been found along the edge of Du Lièvre River, according to Quebec's Ministry of Forests, Wildlife and Parks. In a news release issued Thursday, the ministry said a team of biologists and technicians revisited the area on Wednesday, but found the dead fish had "nearly all disappeared."

That indicates the incident was "sudden and circumstantial," the ministry said.