Four bolts necessary to hold a Boeing 737 MAX’s door plug in place were “missing” before the aircraft was delivered to Alaska Airlines, resulting in a terrifying blowout at 16,000 feet on Jan. 5, according to a preliminary report by the National Transportation Safety Board. Investigators said in the 19-page report that the fuselage, manufactured in Malaysia by Spirit AeroSystems, arrived at Boeing’s Washington state plant on Aug. 31, 2023. Employees soon noted the presence of damaged rivets, a repair job that would have required the removal of the bolts, according to the report. A photo taken in September by workers discussing the work shows three of the four bolts missing; the fourth hole is obscured by insulation, but investigators said laboratory disassembly after the incident proved that its bolt was also gone. It remains unclear why the bolts were not replaced before the plane was delivered to Alaska Airlines. “Whatever final conclusions are reached, Boeing is accountable for what happened,” Boeing chief Dave Calhoun said in a statement. “An event like this must not happen on an airplane that leaves our factory. We simply must do better for our customers and their passengers.”