On the DC-10, the pylon fitting bolts themselves were removed.

"During the investigation, an examination of the pylon attachment points revealed some damage done to the wing's pylon mounting bracket that matched the bent shape of the pylon's rear attachment fitting. This meant that the pylon attachment fitting had struck the mounting bracket at some point. This was important evidence, as the only way the pylon fitting could strike the wing's mounting bracket in the observed manner was if the bolts that held the pylon to the wing had been removed, and the engine/pylon assembly was supported by something other than the aircraft itself. Therefore, investigators could now conclude that the observed damage to the rear pylon mount had been present before the crash occurred rather than being caused by it." NTSB AAR7917

It was bad maintenance procedure. They inspected 3 fleets. United was doing it right. American and Continental were doing it wrong.