CALGARY, Alberta (Reuters) - The Fort McMurray wildfire in northern Alberta that forced the evacuation of 90,000 residents and shut in more than a million barrels per day of oil output was most likely caused by human activity, the Royal Canadian Mounted Police (RCMP) said on Tuesday.

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The RCMP appealed for public assistance in determining how the fire started and whether a criminal offense was involved.

“Wildfire investigators for the province of Alberta have established that the fire was most likely the result of human activity, having ruled out lightning as a probable cause,” the RCMP said in a statement.

Police said they would like to speak with anyone who was in the popular wilderness area known as the Horse River Trail System between April 29 and May 5.

Travis Fairweather, an Alberta Wildfire information officer, said there were a very high number of potential causes of fire linked to human activity, from use of recreational vehicles, to camp fires, industry, power lines, as well as arson.

“They have not narrowed any of that down but they have ruled out lightning at this point,” Fairweather said.

The fire was first spotted by an airborne forestry crew 15 kilometers (9 miles) southwest of Fort McMurray on May 1.

Within 72 hours the blaze had breached city limits and was raging through some neighborhoods, forcing the entire population to hastily evacuate and eventually destroying around 10 percent of structures in Fort McMurray.

Thousands of residents are still displaced and starting the laborious process of rebuilding their homes and businesses.

Around a dozen oil sands projects were forced to shut down operations as a precaution and many are still in the process of ramping back up to full production six weeks later.