Four bolts used to secure the door plug that blew off an Alaska Airlines plane midflight last month appeared to have been removed at Boeing’s factory in Washington last year and never replaced, according to a report issued Tuesday by the National Transportation Safety Board.

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Alaska Airlines Flight 1282 was forced to make an emergency landing on Jan. 5, minutes after taking off from Oregon’s Portland International Airport, after the panel came off the jetliner as it was ascending for a trip to California. The plane was carrying 171 passengers and six crew members at the time.

The door plug and two cellphones blown from the plane were later found in the Portland area, officials said.

According to the NTSB, damage patterns indicate that four bolts that helped to secure the door plug to the frame of the plane appeared to have been missing before the Jan. 5 flight. They had been removed months earlier at Boeing’s facility in Renton, Washington, officials said.

Investigators determined that supplier Spirit AeroSystem installed the door plug in question on the fuselage before shipping it to Boeing in August 2023. After it arrived in Washington, workers found five damaged rivets, and bolts were removed to allow crew members to repair the issue.

Photos reviewed by the NTSB showed that after the repair, at least three bolts appeared to be missing. The fourth bolt presumed to be absent before the Jan. 5 flight could not be seen in the photo, officials said.

Crew members heard a loud bang on the plane as it was climbing shortly after takeoff on Jan. 5, saying it blew open the flight deck door. The captain said his head was pushed into the heads-up display and his headset went up so far that it nearly fell off his head. The first officer’s headset was “completely removed due to the rapid outflow of air from the flight deck,” while flight crew reported that their ears popped.

Flight crew members immediately contacted air traffic control and declared an emergency. The plane landed safely back at Portland International Airport 20 minutes after takeoff.

Seven passengers and a flight attendant suffered minor injuries in the incident.

The Jan. 5 blowout prompted the Federal Aviation Administration to ground similar Boeing 737 Max 9 jetliners to allow for inspections. The planes, which are flown by Alaska Airlines and United Airlines, were cleared to return to service in late January, Reuters reported.

The FAA is also investigating the incident with a focus on Boeing’s safety procedures and compliance with the agency’s regulations.

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NTSB investigates after door plug blown off Alaska Airlines Flight 1282 In this National Transportation Safety Board (NTSB) handout, oxygen masks hang from the fuselage as members of the NTSB examine Alaska Airlines Flight 1282, a Boeing 737 Max 9, on Saturday, Jan. 7, 2024 in Portland, Oregon. A door-sized section near the rear of the plane blew off 10 minutes after Alaska Airlines Flight 1282 took off from Portland, Oregon on January 5 on its way to Ontario, California. (Handout/NTSB via Getty Images)

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