The United States Postal Service (USPS) is the latest target of a card skimming scheme that has affected at least 13 states and the District of Columbia.
Customers who used the post offices' self-service stamp vending machines reported multiple fraudulent credit card purchases, according to technology journalist Brian Krebs.
The USPS's Inspection Service opened an investigation into the scheme, but haven't provided details. However, the agency told its employees to begin inspecting the machines throughout the day.
The fraudsters stole PINS and magnetic strip data from the cards to create new cards and withdraw cash at ATMs. The scheme is thought to have begun in November 2013.
Customers who notice anything unusual are being asked to report their suspicions to the local post office supervisor.