DULUTH, MN (KDAL) - The Duluth Fire Department responded to a structure fire at 4201 West 6th Street around 6:45 a-m on Tuesday. When they arrived heavy smoke was coming from the upstairs apartment. The tenants were alerted by a smoke alarm and attempted to put out the fire with a extinguisher but were unsuccessful. They left the apartment and called 911. Damage to the structure and contents are estimated at over 50 thousand dollars. The cause of the fire is undetermined and will be under investigation by the Duluth Fire Marshal's office.