First, there was a pop. And then a big bang. Air loudly whooshed out of the side of the airplane, which was flying at 16,000 feet with an emergency exit size gash. A cellphone, a teddy bear, and a passenger’s shirt were sucked out the hole in the cabin. Oxygen masks dropped from overhead compartments.

Passengers on Alaska Airlines flight 1282—which was enroute to Ontario, Calif., from Portland, Oregon—were fearful for their lives. The flight, however, landed back at the Portland airport less than 30 minutes after takeoff, with 171 passengers and six crew members aboard, all of them alive.

One passenger said, “We literally thought we were going to die.” Bolts needed to secure part of an Alaska Airlines jet that blew off in midair appear to have been missing when the plane left the factory.

Boeing and other industry officials increasingly believe the plane maker’s employees failed to put back the bolts when they reinstalled a 737 MAX 9 plug door after opening or removing it during production.

The scenario was based partly on an absence of markings on the Alaska door plug itself that would suggest bolts were not in place when it blew off the jet around 16,000 feet over Oregon. They also pointed to paperwork and process lapses at Boeing’s Renton, Washington factory related to the company’s work on the plug door.