LONDON — A criminal investigation has been launched after the U.S. Air Force reported that drones were spotted flying over English military bases, a British official said Wednesday.

Unidentified drones are still being spotted over United States Air Force (USAF) bases in England on Wednesday, a USAF spokesperson said, confirming that a fourth base had also been targeted by the week-long airspace incursions.

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.”