PORTSMOUTH, Va. — A series of threatening messages has been found in restrooms at a U.S. Navy shipyard in Virginia.

The Virginian-Pilot cites a Friday release from the Navy that says four messages had been found over the last 10 days at Norfolk Naval Shipyard. Shipyard spokeswoman Terri Davis says the first message was a bomb threat found May 22.

The messages were found in bathrooms across the shipyard, including one portable bathroom.

The Naval Criminal Investigative Service is offering a $2,500 reward for information leading to prosecution.