(CNN) — Investigators from the National Transportation Safety Board on Tuesday said evidence shows four bolts that hold the door plug in place on the Boeing 737 Max 9 were missing at the time of last month’s blowout on Alaska Airlines flight 1282.

The bombshell new finding from federal investigators comes one month and a day after the January 5 incident that triggered a 19-day emergency grounding of all Max 9s, and re-ignited scrutiny of Boeing following the fatal Max 8 crashes of 2018 and 2019.

In their 19-page preliminary report released Tuesday, NTSB investigators included observations from a laboratory disassembly of Alaska 1282’s door plug, which fell 16,000 feet into an Oregon backyard. It said the lack of damage to the plug where the bolts were supposed to attach it to the fuselage of the plane pointed to the conclusion that the bolts were missing at the time of the flight.

“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 [mid exit door] plug moved upward off the stop pads,” the report said referring to the mid exit door.

Six crew and 171 passengers were on board the flight, which returned safely to Portland International Airport. Nobody was seriously hurt in the incident.

The missing bolts are apparently not the only problem. Both Alaska Airlines and United Airlines said last month that inspections of their fleets that took place after the January 5 incident revealed loose bolts.

“This is a somewhat complex issue with a lot of parts,” NTSB Chair Jennifer Homendy told CNN in the week leading up to the release of the preliminary report. Even still, Homendy stressed that she “would have no problem getting on a Max 9 tomorrow and flying.”

The release of the report comes as Boeing’s quality control is under intense scrutiny. During a House subcommittee hearing Tuesday, FAA Administrator Mike Whitaker said there are now two dozen FAA inspectors on location at Boeing’s Renton, Washington plant as part of an agency audit.

Late Sunday, Boeing disclosed that it would need to “rework” improperly drilled holes discovered on 50 incomplete 737 Max planes still on the production line, causing a slowdown in deliveries.

Later, Boeing fuselage contractor Spirit AeroSystems said it had caused the problem.

The preliminary report does not lay blame on Boeing nor find a probable cause which is typically included in the NTSB’s final report which could take a year or more.

The report also details the shock of the blowout, which caught passengers and crew by surprise.

“The captain said that, while climbing through about 16,000 ft, there was a loud bang,” the report said. “The flight crew said their ears popped, and the captain said his head was pushed into the heads-up display (HUD) and his headset was pushed up, nearly falling off his head. The FO [first officer] said her headset was completely removed due to the rapid outflow of air from the flight deck.”

CNN has reported that NTSB investigators have been closely scrutinizing the door plug and whether crucial bolts that hold it in place were properly installed when the incident occurred.

NTSB preliminary reports do not determine the cause of incidents but can reveal key facts about what took place.

Meanwhile, the head of the Federal Aviation Administration told House lawmakers Tuesday that his agency is “closely scrutinizing” Boeing after last month’s door plug blowout.

“Going forward, we will have more boots on the ground closely scrutinizing and monitoring production and manufacturing activities,” FAA Administrator Mike Whitaker said in prepared remarks.

The Federal Aviation Administration failed to properly scrutinize Boeing after two fatal crashes of 737 Max aircraft killed 346 people more than four years ago, the agency’s new chief said.

“I wasn’t there at the time as you noted. I guess I would say in retrospect and given what happened with the plug door, it’s hard to call that oversight sufficient,” Whitaker told Congress on Tuesday.  “So, we’re looking at that process and what additional steps need to be taken to make sure that oversight is sufficient.”

Whitaker was sworn in late last year and was not at the FAA at the time of those crashes in 2018 and 2019.  He was previously the second-ranking FAA official from 2013 to 2016. Whitaker appeared before the House aviation subcommittee, his first congressional testimony since being confirmed to the post little more than three months ago.

His agency is currently overhauling how it scrutinizes plane manufacturers, including Boeing, after a hole blew open in the side of a 737 Max 9 last month.

“I certainly agree that the current system is not working cause it is not delivering safe aircraft,” Whitaker said. “We have to make some changes to that. And I think we also have to look at the culture.”

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