The investigation into the midair blowout of a fuselage door plug on Alaska Airlines Flight 1282 on Jan. 5 has confirmed that four bolts that should have kept the door plug in place were missing when Boeing delivered the aircraft, the National Transportation Safety Board said Tuesday in a preliminary report on the incident. “Four bolts that prevent upward movement of the mid exit door plug were missing before the mid exit door plug moved upward off the stop pads,” the report states. The NTSB said the door plug was opened at Boeing’s Renton, Washington, factory so a team from supplier Spirit Ae…