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

Small, unmanned craft were seen flying over four bases in England, the U.S. Air Force in Europe said. The Pentagon said that there had not been “any significant mission impact.”

Unidentified drones have been spotted again over three British air bases used by the U.S. Air Force, officials confirmed Tuesday. The three bases are all located within the same area of eastern England between Cambridge and Norwich,