The U.S. National Transportation Safety Board (NTSB) said on Tuesday in its preliminary investigative report that four key bolts were "missing" from a door panel of an Alaska Airlines-operated 737 Max 9 that blew off in mid-air on January 5, which triggered a 19-day grounding of all Boeing Max 9 models.

"Overall, the observed damage patterns and absence of contact damage or deformation around holes associated with the vertical movement arrestor bolts and upper guide track bolts in the upper guide fittings, hinge fittings, and recovered aft lower hinge guide fitting indicate that four bolts that prevent upward movement of the MED plug were missing before the MED plug moved upward off the stop pads," the NTSB said in the report.

The door plug was originally installed by contractor Spirit AeroSystems in Kansas. When it was shipped to Boeing's factory in Renton, Washington, for assembly, damaged rivets were discovered on the fuselage that required the door plug to be opened for repairs.

After repair work was completed by Spirit AeroSystems personnel at the Boeing plant, the bolts were not reinstalled, according to photo evidence provided to the NTSB by Boeing.

Boeing said that it appreciates the NTSB's work and will review their findings expeditiously.

"We will continue to cooperate fully and transparently with the NTSB and the FAA (Federal Aviation Administration) investigations," the company said in a statement.

"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. We simply must do better for our customers and their passengers," Boeing President and CEO Dave Calhoun said.

According to Calhoun, Boeing is implementing a comprehensive plan to strengthen quality with "significant, demonstrated action and transparency at every turn."

The company has implemented a control plan to ensure all 737-9 mid-exit door plugs are installed according to specifications, Boeing said, adding that it is implementing plans to improve overall quality and stability across the 737 production system.

"In addition to these Boeing actions, we are opening our factory to 737 customers to conduct their own additional reviews, and will fully and transparently support the FAA's investigation, audit and oversight actions," Calhoun noted.

"This added scrutiny – from ourselves, from our regulator and from our customers – will make us better. It's that simple," he said. 

(With input from Xinhua)