The U.S. National Transportation Safety Board said on Monday it will investigate a close call between a Delta Air Lines plane and a group of Air Force jets near Reagan Washington National Airport last week.

Four jets came close enough to the Delta flight, which was taking off from Washington’s Reagan National Airport, to set off a collision-avoidance alert.

The National Transportation Safety Board (NTSB) said on Monday it is investigating the close call between a commercial Delta Air Lines flight and a group of Air Force jets flying near Reagan