An "incendiary device" placed inside a ballot box sparked a fire early Monday in Portland, Oregon, police say. 

The Portland Police Bureau said officers responded "to reports of a fire at a ballot box in the 1000 block of Southeast Morrison Street" around 3:30 a.m. local time. 

"By the time officers arrived, the fire had already been extinguished by security personnel who work in the area. Officers determined an incendiary device was placed inside the ballot box and used to ignite the fire," police said in a statement. "PPB’s Explosive Disposal Unit responded to the scene and cleared the device." 

It is unclear if any ballots were damaged in the fire. A detective investigating the case did not immediately respond Monday to a request for comment from Fox News Digital. 

ARIZONA ELECTION BALLOTS DAMAGED AFTER USPS MAILBOX LIT ON FIRE, SUSPECT ARRESTED 

"The Portland Fire Investigations Unit also responded to the scene to investigate. This unit is comprised of fire investigators with Portland Fire & Rescue, who are sworn law enforcement officers, and a detective with the Portland Police Bureau," police added. "The collaboration leverages the specialized training and skills needed to investigate fires, including analysis equipment and accelerant detecting dogs." 

Police are describing the incident as an "arson case." 

MILLIONS OF VOTERS HAVE ALREADY CAST BALLOTS FOR NOV. 5 ELECTION 

It happened just days after about 20 mail-in ballots were damaged when a U.S. Postal Service mailbox was lit on fire in Phoenix.

Dieter Klofkorn, 35, has since been arrested on one felony count of Arson to Property. 

"Klofkorn stated that he committed the arson because he wanted to be arrested and that his actions were not politically motivated and not related to anything involving the upcoming election," the Phoenix Police Department said in a statement describing the incident last Thursday. 

Images taken from the scene of that incident showed a charred mailbox outside a U.S. Post Office in Arizona’s capital city.