WASHINGTON, Feb 6 (Reuters) – 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 have demanded answers to what caused the panel to flyoff a brand-new Alaska Airlines-operated jet, in what has turned into a full-blown safety and reputational crisis for Boeing.

The U.S. National Transportation Safety Board (NTSB) 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 boltsand then secured by “stop fittings” at 12 different locations along the side of the plug and the door frame.

Representative Rick Larsen, the top Democrat on the committee overseeing the FAA, said the “failure to re-install bolts on a safety-critical component of this 737 MAX 9 aircraft is a serious error that signals larger quality control lapses that must be corrected.”

Photo Rob Vogelaar