(Portland, OR) -- A large pile of shredded tires caused a large fire Monday morning in North Portland. Portland Fire & Rescue crews responded to North Lombard and Harborgate Street where the fire was sending a column of dark smoke over the area. The pile of shredded rubber is located next to Union Pacific railroad tracks. Firefighters worked to keep the flames from spreading to nearby buildings. A bulldozer from the business helped to separate the material, so water could put out the fire. The investigation into the cause of the fire is ongoing. Surveillance video is being reviewed for potential suspicious activity.