A preliminary NTSB report found that four bolts meant to hold a door plug panel in place were missing from a Boeing 737 Max 9 plane flown by Alaska Airlines. The bolts had to be removed at Boeing's factory last year to replace damaged rivets, but photos show the panel was reinstalled without the bolts. A former Boeing manager said missing a bolt indicates a manufacturing process breakdown, and employees may have felt rushed. The NTSB investigation is ongoing.