MINNEAPOLIS (WCCO) — A grass fire now covers almost 13,000 acres in northwestern Minnesota.

It started burning on Monday in Glacier Ridge National Wildlife Refuge near Mentor. Fire crews had to temporarily close some highways in that area.

A still image from dash cam video of the Mentor fire (credit: MN State Patrol)

Crews have most of the fire under control as of Thursday evening. Investigators say it was started by humans, but they still don’t know how.

READ MORE: 7 Benton County Grass Fires May Have Been Intentionally Set