The NTSB is carrying out a full investigation. This will involve, for example, examining the plane itself, the lost door (which has now been found in a back garden in Oregon), the contents of the flight data recorder and maintenance records, as well as interviewing flight crew, maintenance staff and employees of Boeing and Spirit AeroSystems.
https://www.bbc.co.uk/news/world-us-canada-67915771?at_medium=RSS&at_campaign=KARANGA,