Over the past week, unidentified drones have been seen flying over joint U.S.-U.K. military bases in the United Kingdom.

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.”

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