(NewsNation) —A preliminary report by the National Transportation Safety Board states that there were bolts missing from a door plug that blew off of the side of a Boeing 737 Max 9 plane during a Jan.5 Alaska Airlines Flight.

The plane had been flying over Oregon at the time. Only minor injuries were reported.

Following this incident, the FAA grounded all Max 9s in the United States for nearly three weeks, forcing Alaska and United Airlines to cancel more than 1,000 flights between them.

Released Tuesday, the NTSB report does not determine the cause of the blowout, but it can reveal key facts about what happened.

In the report, it says that when the plane was at 16,000 feet, the captain heard a loud bang. His headset was pushed up, nearly falling off his head, the report said, while the flight crew immediately donned oxygen masks.

Because the flight deck door was open, it was “very noisy and difficult to communicate,” the report said.

Crew members immediately contacted air traffic control, declared an emergency and requested a lower altitude. The captain was then able to fly the plane and it landed and taxied to the gate without incident.

Photo documentation from Boeing shows that bolts, meant to hold the door plug in place, were missing and rivets were damaged.

“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 the four bolts that prevent upward movement of the MED plug were missing before the MED plug moved upward off the stop pads,” the report said.

Boeing said in a statement that it will review the NTSB’s findings “expeditiously,” and will cooperate fully with the agency and the Federal Aviation Administration.

“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,” Boeing CEO Dave Calhoun said. “We simply must do better for our customers and their passengers. We are implementing a comprehensive plan to strengthen quality and the confidence of our stakeholders. It will take significant, demonstrated action and transparency at every turn – and that is where we are squarely focused.”

In their own statement, Alaska Airlines said its safety and technical representatives, as well as Boeing’s, remain in close contact with the NTSB.

“Safety is always our top priority. As this investigation moves forward, we have full confidence in the safety of our operation and aircraft,” Alaska Airlines said.

FAA Administrator Mike Whitaker has said the administration will not grant any production expansions of the MAX aircraft until it is “satisfied that the quality control issues uncovered during this process are resolved.”

Whitaker, speaking in front of a House subcommittee Tuesday, the FAA is halfway through a six-week audit that includes placing “about two dozen” inspectors in Boeing’s 737 plant in Renton, Washington, and “maybe half a dozen” at a Wichita, Kansas, plant where upplier Spirit AeroSystems makes the fuselages for 737s.

You can read the full NTSB report here:

This story is developing. Refresh for updates.

The Associated Press contributed to this report.