Video shows the aircraft on fire and a fiery fragment separates from it in that quick few seconds before it hits the ground. I think an engine’s turbine fragmented and just blew up.
That would be consistent with the fire seen on the descending plane, and if the turbine failure was violent enough it could have destroyed the second engine, and damaged the tail of the aircraft. On a Learjet 55 the engines are located next to each other just in front of the tail.