Evidence from the door that blew off an Alaska Airlines plane is bolstering a theory that the emergency occurred because bolts required to secure the door were missing when the aircraft left Boeing’s factory, according to The Wall Street Journal. The so-called door plug on the 737 Max 9 aircraft blew out at around 16,000 feet during the flight on Jan. 5 and was later discovered in a teacher’s backyard. Now Boeing and other industry officials “increasingly believe” that the aircraft manufacturer’s workers did not put bolts back in place when they reinstalled the door after removing it or opening it during the production, sources told the Journal. The conclusion is partly based on an apparent lack of markings on the door that would indicate the bolts were in place at the time it blew off, according to the report.