Police in northern Alberta are asking for help from the public, as they investigate the wildfire that forced the evacuation of tens of thousands of people more than a month ago.

RCMP said investigators had been working with provincial wildfire investigators to determine the cause of the fire that started fifteen kilometres southwest of Fort McMurray in early May, and grew to prompt the evacuation of that city, and surrounding communities.

Investigators have determined the fire was likely started by humans, and have ruled out lightning as a possible cause. The fire was first reported by an airborne forestry crew on May 1.

At this point, police said they haven’t determined how the fire was started, but want to speak with anyone who was in the area of the Horse River Trail System between April 29 and May 5 – it’s believed someone there at that time might have information that could help police.

RCMP said the role of police investigators is to determine if a criminal offence was behind starting the fire.

Anyone with information is asked to call the RCMP’s dedicated wildfire investigation line at 1-844-620-9826.

On Monday, wildfire officials said that fire was listed as 'being held', after it burned out of control for more than a month.