The U.S. National Transportation Safety Board on Tuesday is questioning key witnesses from Boeing, Spirit AeroSystems and the Federal Aviation Administration on the mid-air cabin panel blowout of an Alaska Airlines 737 MAX 9 door plug in January.

During the start of its two-day investigative hearing, the board released 3,800 pages of factual reports and interviews from the ongoing investigation.

The incident badly damaged Boeing’s reputation and led to the MAX 9 being grounded for two weeks, a ban by the FAA on expanding production, a criminal investigation and the departure of several key executives. Boeing has vowed to make key quality improvements.

Boeing’s senior vice president for quality, Elizabeth Lund, and Doug Ackerman, vice president of supplier quality for Boeing, are testifying Tuesday during the hearings scheduled to last 20 hours over two days. Ackerman said Boeing has 1,200 active suppliers for its commercial airplanes and 200 supplier quality auditors.

Lund said Tuesday Boeing is still building “in the 20s” for monthly MAX production – far fewer MAXs than the 38 per month it is allowed to produce. “We are working our way back up. But at one point I think we were as low as eight,” Lund told the NTSB.

A flight attendant described a moment of terror when the door plug blew out. “And then, just all of a sudden, there was just a really loud bang and lots of whooshing air, like the door burst open,” the flight attendant said. “Masks came down, I saw the galley curtain get sucked towards the cabin.”

Terry George, senior vice president and general manager for Boeing Program at Spirit AeroSystems, and Scott Grabon, a senior director for 737 quality at Spirit, which makes the fuselage for the MAX, also testified Tuesday.

Last month Boeing agreed to buy back Spirit AeroSystems, whose core plants it spun off in 2005, for $4.7 billion in stock.

The hearing is reviewing issues including 737 manufacturing and inspections, safety management and quality management systems, FAA oversight, and issues surrounding the opening and closing of the door plug.

Boeing has said no paperwork exists to document the removal of four key missing bolts. Lund said Boeing has now put a bright blue and yellow sign on the door plug when it arrives at the factory that says in big letters: “Do not open” and adds a redundancy “to ensure that the plug is not inadvertently opened.”

In June, FAA Administrator Mike Whitaker said the agency was “too hands off” in its oversight of Boeing before January. FAA employees told the NTSB that Boeing employees did not always followed required processes.

Jonathan Arnold, Aviation Safety Inspector at the FAA, said a systemic issue he witnessed at Boeing’s factory was employees not following the instructions.

“That seems to be systemic where they deviate from their instructions. And typically, tool control is what I see most,” Arnold said.

Lund said before the Jan. 5 accident, every 737 fuselage delivered to Boeing had defects – but the key is making sure they are manageable. “What we don’t want is the really big defects that are impactful to the production system,” Lund said. “We were starting to see more and more of those kinds of issues. I will tell you right around the time of the accident.”

The interviews also addressed questions of factory culture, which has been under fire in congressional hearings. Whistleblowers have alleged that Boeing retaliated against people coming forward with safety concerns on the factory floor.

Boeing executive Carole Murray described various problems with fuselages coming from Spirit AeroSystems in the run-up to the accident. “We had defects. Sealant was one of our biggest defects that we had write-ups on,” she said. “We had multiple escapements around the window frame, skin defects.”

Michelle Delgado, a structures mechanic who worked as a contractor at Boeing and did the rework on the Alaska MAX 9 aircraft, told NTSB the workload is heavy and requires working long hours.

“When we’re very overwhelmed with work, it is pressing because with everything we’ve cut down on some personnel, so now it’s like in order for me to not have to deal with a worse situation tomorrow, I’d rather work a 12 to 13-hour shift to get it all done, for my sake, so I don’t have to deal with people the next day.”