A ballot box in Portland, Oregon, was set on fire with an incendiary device Monday, police said.

Portland police responded to reports of a fire at a ballot box around 3:30 a.m. local time on Monday. Security personnel who worked in the area were able to extinguish the fire, police said.

"Officers determined an incendiary device was placed inside the ballot box and used to ignite the fire," the Portland Police Bureau said in a statement.

The bureau's explosive disposal unit cleared the device, police said.

Police did not say how many ballots were impacted by the fire.

The Portland Fire Investigations Unit is investigating.