> Ground observers reported the aircraft had been delayed for about two hours for work on the left hand engine (engine #1), the engine #1 separated during the takeoff run, the center engine emitted streaks of flames, the aircraft impacted a UPS warehouse and ploughed through other facilities before coming to rest in a large plume of fire and smoke.
So the tl'dr is: the leading very preliminary theory is that the MD-11's left engine fell off the wing just like https://en.wikipedia.org/wiki/American_Airlines_Flight_191 (a DC-10, the immediate predecessor of the MD-11) which was caused by maintenance errors weakening the pylon structure holding the engine.
1) improper maintenance—American Airlines had used a forklift shortcut to remove the engine and pylon together, rather than following McDonnell Douglas’s prescribed method
2) The detachment tore away part of the wing’s leading edge, rupturing hydraulic lines and severing electrical power to key systems, including the slat-position indicator and stall warning (stick shaker).
3) The pilots followed the standard engine-out procedure and reduced airspeed to V₂, which caused the aircraft to stall and roll uncontrollably left. This procedure was later found out to be incorrect.
Defective maintenance practices, inadequate oversight, vulnerabilities in DC-10 design, and unsafe training procedures combined to cause the crash, killing all 273 people on board and leading to sweeping reforms in airline maintenance and certification standards.
(I do not mean to imply that this exact slat retraction is necessarily relevant in the Louisville crash, however - I believe aircraft since AA191 are designed to maintain their wing configuration after loss of hydraulic pressure.)