Homeowners in the Heatherstone Subdivision near Cary received a white piece of paper stating "Project Mayhem has begun."
Homeowners said they are not sure if the letter is a hoax, a Halloween prank or a serious threat.Concerned residents contacted the Wake County Sheriff's Office and the postal inspector.
Capt. Patricia Bell of the Wake County Sheriff's Office said there have been complaints of vandalism in the neighborhood in the past, including broken windows and damaged mailboxes.
Officials believe teens are behind the prank, but are not taking the situation lightly. More officers are patrolling the neighborhood looking for anything suspicious.
For the most part, neighbors said they are not too concerned.
"There's so much going on right now in the world, I think people are trying to stir up things on a local level to get some attention," resident Mari Hoglund said.
Tampering with a mailbox is a federal offense.