Around 20 election ballots have been burned in Phoenix, Arizona during a suspected arson incident targeting a U.S. Postal Service mailbox, officials say.

The individual allegedly set fire to the mailbox early Thursday morning at a Post Office at 3905 North 7th Avenue, the Phoenix Fire Department wrote in a statement to Newsweek. Firefighters forced the box open and put out the fire, finding that ballots and other mail was damaged.