WASHINGTON -The U.S. Federal Aviation Administration on Saturday issued an order prohibiting MD-11 planes from flying for now, pending inspection, after a fiery crash this week of a UPS MD-11 cargo freighter killed at least 14 people in Louisville, Kentucky.

UPS and FedEx said they have already grounded their combined fleet of more than 50 McDonnell Douglas MD-11 cargo planes following a recommendation by planemaker Boeing.

The FAA said its emergency airworthiness directive was prompted by the crash on Tuesday in which the left engine and pylon detached from the airplane during takeoff. The cause of the detachment is under investigation.

“This condition could result in loss of continued safe flight and landing,” the U.S. regulator said.

UPS said it was in compliance with the directive because it has grounded its fleet of 26 MD-11 planes, representing about 9% of its fleet.

Boeing said on Saturday that it supports the FAA order.

Boeing acquired the MD-11 program through its 1997 merger with McDonnell Douglas.

Officials said the FAA is likely to require comprehensive engine and pylon inspections before allowing the planes to resume service.

The U.S. National Transportation Safety Board is leading the probe into the crash of the 34-year-old MD-11 freighter at the airport. The plane reached about 100 feet (30.5 meters) above ground level before plunging in flames off the runway and destroying two nearby businesses.

One of the plane’s three engines detached from the left wing as it rolled down the Louisville airport runway.

On Friday, U.S. safety investigators said that as a warning bell sounded in the cockpit, three UPS pilots had tried to wrestle control of the plane just before it crashed.