A door panel that flew off a Boeing 737 MAX 9 jet mid-flight was missing four key bolts, according to a preliminary report from US investigators.
The Alaska Airlines jet had just left on its way to Ontario, California when the plug blowout occurred at 16,000 feet.
The panel covering an unused emergency exit, known as the door plug in the aviation industry, blew out leaving a hole in the side of the plane.
Photographic evidence released on Tuesday shows bolts were missing from the door plug, according to the independent US National Transportation Safety Board report.
The US Federal Aviation Administration (FAA) grounded Boeing 737 MAX 9 planes after the incident which happened in January.
Boeing President and CEO Dave Calhoun said after the preliminary findings: “Whatever final conclusions are reached, Boeing is accountable for what happened. An event like this must not happen on an airplane that leaves our factory."
Several passengers sustained minor injuries but were treated by paramedics and did not require hospital treatment.
There were 171 passengers in total, in addition to four flight attendants and two captains.
No had been seated next to the plug, which can sometimes double as an emergency exit.
The door blowout lead to rapid depressurization, causing oxygen masks to fall from the vents and debris from the plane, including two iPhones, to be hurled out.
The door plug was later found in a teacher’s backyard, while the two phones were found in streets below.
Testifying before US lawmakers, the head of the FAA, Mike Whitaker, said inspections of 737 Max aircraft had shown that "the quality system issues at Boeing were unacceptable and require further scrutiny".