The National Transportation Safety Board (NTSB) has released a preliminary report on the Alaska Airlines Boeing Max 9 midair blowout last month, finding that four key bolts were missing from the door plug.

"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 reads.

The 19-page report on Alaska Airlines flight 1282 comes one month and a day after the flight was forced to make an emergency landing on Jan. 5, shortly after taking off from Oregon’s Portland International Airport.

A door plug blew off the jetliner as it was ascending for a trip to California. 

No injuries were reported on the plane, which was carrying 171 passengers and six crew members at the time.

The Jan. 5 blowout prompted the Federal Aviation Administration to ground similar Boeing 737 Max 9 jetliners to allow for inspections, which resulted in thousands of flight cancellations.

Government agencies investigated potential hazards that could affect flights from Alaska Airlines and United Airlines, the two U.S. air carriers that use the 737 MAX 9, but those planes have since been cleared for travel again as of Jan 24. 

The head of the U.S. Federal Aviation Administration, Mike Whitaker, spent Tuesday morning on Capitol Hill speaking to lawmakers and emphasizing that the current system is not working. 

"I certainly agree that the current system is not working, because it's not delivering safe aircraft," Whitaker said. "So we have to make changes to that."

United Airlines announced last month that it found loose bolts on its Boeing 737 MAX 9 planes, days after an Alaska Airlines plane of the same model lost a door plug mid-air.

This is a developing story.