A squadron of mystery drones has been buzzing over Langley Air Force Base in Virginia, one of the most heavily guarded military zones in the United States, for 17 straight nights, confounding Pentagon officials who are unable to determine their origin or purpose.

The Pentagon has confirmed a number of “unauthorized” drone flights last year in restricted airspace over a Virginia base that houses the nation’s most advanced fighter jets. The Wall Street

U.S. officials don’t know who is behind the drones that have flown unhindered over sensitive national-security sites—or how to stop them.