Probe suggests Boeing jet panel that blew out mid-flight was missing bolts

A door panel that flew off a Boeing 737 MAX 9 jet mid-flight on Jan. 5 appeared to be missing four key bolts, according to a preliminary report from U.S. investigators that provided the first official look into how the frightening mishap took shape.

Lawmakers and the flying public are desperate for answers to what caused the panel to rip off a brand-new Alaska Airlines-operated jet, in what has turned into a full-blown safety and reputational crisis for Boeing.

“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," said Boeing President and CEO Dave Calhoun.

The U.S. Federal Aviation Administration grounded 171 of the Boeing 737 MAX 9 planes after the incident, most operated by U.S. carriers United Airlines and Alaska Airlines, for inspections. Those planes were cleared to return to service in late January.

The U.S. National Transportation Safety Board report released on Tuesday focused on how the panel - fitted into this MAX 9 model in place of an optional exit - could have detached from the plane. The plug is held down by four bolts and then secured by "stop fittings" at 12 different locations along the side of the plug and the door frame.

The report raises questions about who initially installed the bolts and why the door's opening at Renton to correct the rivets was not properly documented, said U.S. aviation safety expert John Cox.

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