A British government official says a criminal investigation is underway after drones were spotted recently by the U.S. Air Force over English military bases

The drones were detected between Wednesday and Sunday, near Royal Air Force Lakenheath, Royal Air Force Mildenhall and Royal Air Force Feltwell, all three of which are in eastern England. The bases house thousands of U.S. service members, primarily Air Force personnel.

The Air Force didn’t identify who was behind the incursions, but said officials determined there was no impact on residents or critical infrastructure.