Arizona law enforcement officials are conducting an arson investigation after election ballots were damaged when a USPS mail collection box was burned in Maricopa County, a Democratic stronghold.

ABC 15 reported that crews responded to the USPS Osborn Station around 1:30 a.m. on Thursday and found a mail collection box on fire.

“Approximately 20 electoral ballots were damaged, along with additional miscellaneous mail,” Phoenix Fire Department officials revealed, according to ABC 15.