The finding was released by the National Transportation Safety Board on Thursday.
Sixteen passengers suffered minor injuries during Flight 345's landing Monday. The plane was coming from Nashville, Tenn.
A spokeswoman for Southwest said the landing described by the NTSB was "not in accordance" with operating procedures.
The NTSB had said Wednesday that the landing gear "collapsed rearward and upward into the fuselage, damaging the electronics bay that houses avionics."
Investigators recovered the flight data and cockpit voice recorders on Tuesday. They'll be analyzed by the NTSB.