A door on an Alaska Airlines flight in January broke off, leaving a gaping hole in the side of the Boeing 737 Max, and the regulator held the first two days of hearings into the incident on Tuesday. Anna Moneymaker/Getty Images

Washington, USA – Seven months of investigation. Nearly 10 hours of hearings. And yet neither Boeing nor the U.S. National Transportation Safety Board (NTSB) know how a 737 Max was delivered to Alaska Airlines without the four bolts needed to hold the door seal in place.

What’s known: Boeing’s procedures and training — from employees and safety researchers — have drawn skepticism and intense criticism from regulators. That much became clear on the first day of a two-day hearing. The NTSB called a rare public hearing to look into the near-miss of an Alaska Airlines flight on Jan. 5, in which a door plug broke, ripping a hole in the side of the plane — and Boeing’s already damaged reputation.

The door plug was removed at Boeing’s Renton, Wash., factory last September so problems with some of the screws could be fixed. But documentation for the temporary removal of the door plug apparently wasn’t created. When workers temporarily replaced the door plug, other workers were unaware that the screws needed to be reinstalled, said Elizabeth Lund, senior vice president of quality for Boeing Commercial Airplanes.

But under NTSB questioning, Lund admitted it was unclear who or when the door plug was installed. That lack of information concerns NTSB members.

“We don't know and they don't know, and that's a problem,” NTSB Chair Jennifer Homendy told reporters during a break in the session.

The second day of hearings focused on Boeing's safety management system and quality management systems that have been in place for several years and are supposed to prevent problems.

Boeing executives said the two programs were working to improve work done at their factories, though they acknowledged that the January accident showed there were “gaps” in the systems and that improvements still needed to be made.

Homendy told CNN that the Boeing 737 factory has had problems with “unauthorized” work on door handles “for many years,” so she is concerned that the January 5 incident could happen again.

“Could this happen again? Of course,” Homendy said. “Yes, there are more inspections, but what can stop this? You can’t rely on just one person or a group of people.”

Homendy also suggested the problem could happen again while questioning Boeing executives.

He asked, “I want to ask you, are you 100% sure that the glitch will not happen tomorrow? Yes or no?”

“No,” replied Hector Silva, vice president of regulatory compliance and core quality at Boeing.

“Are you 100% sure there won’t be any unauthorized removal (of the port plug)?” Homendy asked.

“No,” Silva replied.

When asked if he could promise that no other door plug would be installed incorrectly, Silva replied, “I can't promise or guarantee that. All I can say is that we are absolutely committed to making sure that all necessary changes are made.”

Other NTSB members expressed dissatisfaction with Boeing and the FAA's oversight of the company.

“Is it just me, or do we see a word game every five or 10 years about safety?” said board member J. Todd Inman, citing previous agreements between Boeing and the FAA that were supposed to resolve quality issues. “What do you think will be different this time?”

Homendy said Boeing disclosed that two employees who worked on the Alaska Airlines plane were placed on “leave” with pay as part of the investigation. He criticized the move, saying it sends a message that there will be retaliation even for unintentional mistakes and will discourage other workers from speaking out when they learn of wrongdoing, for fear of being retaliated against.

“Safety is a collective responsibility. It’s not just one person. It’s not just two people,” Homendy said. “So I have a lot of questions about those two people who were removed. I just want to understand why two people on the door team were sidelined for something that is a collective responsibility.”

To avoid this problem in the future, Boeing is considering adding a warning light in the cockpit to alert pilots if the door leaf moves even slightly — long before it explodes in a similar incident on an Alaska Airlines flight.

All 737 exit doors have an indicator light of this type in the cockpit if one of the aircraft's doors is out of the locked position. But since the door covers are not intended to be opened and closed, except for maintenance purposes, they are not equipped with the same function. However, this change will likely take about a year to implement and should be available to be retrofitted to fit existing aircraft with door plugs.

Meanwhile, Boeing has revealed that it has a far less technological response to ensure that aircraft door seals aren’t removed at the factory and then reinstalled without the necessary screws: The company attaches a laminated blue and yellow sticker to all door seals. When the doors arrive at the factory, they are labeled “DO NOT OPEN” in relatively large letters. And in small print, “DO NOT CONTACT QUALITY ASSURANCE.”

Workers express concern

Testimony from Boeing workers before NLRB regulators, released as part of Tuesday’s hearings, showed workers questioning the training they were required to receive for initiatives like reinstalling door seals and other changes to aircraft. They also complained of constant pressure to speed, of planes arriving on the assembly line riddled with defects, and of treating supplier employees at Boeing factories like “cockroaches.”

Overall, the testimony depicts a company in disarray, with little training and sometimes confusion about who was doing what. In short, the hearings held so far paint a picture of a company that has yet to recover from a series of safety failures that left everyone from regulators to ordinary passengers in shock—failures serious enough for the company to agree to plead guilty to defrauding the Federal Aviation Administration. Boeing could face further criminal charges related to the Alaska Airlines incident, and the plea deal will force the company to operate under federal control for years.

Boeing executives and supplier Spirit AeroSystems have gone to great lengths to assure the NTSB that they have made changes to their operations that would prevent another tragedy.

“We feel this is not going to be a recurring trend,” Lund said, pointing to improved metrics, increased training and inspections since the Jan. 5 incident, and promising that the changes at Boeing would be permanent.

“These things will not be taken away,” he added.

But she and other Boeing executives faced tough questions and criticism from board members.

“I just want to say a word of caution here. This is not a public relations campaign for Boeing,” Homendy said at one point during the hearing, chiding the company for focusing too much on what has happened since the accident and not enough on the problems that allowed the accident to happen.

“You can talk about your position today, there will be time for that,” she said. “This is an investigation into what happened on January 5. Do you understand?”

As part of the public hearing, the NTSB released 70 documents covering nearly 4,000 pages, filled with disturbing statements from Boeing workers and other experts, including from the Federal Aviation Administration, about the problems at Boeing.

The workers, many of whom were not identified in the transcripts released by the NTSB, described being forced to do more work than they could do without making mistakes, and of problems as one plane after another moved along Boeing's assembly lines, with many of them regularly needing to be reworked.

One worker told NTSB investigators that the plane problems put workers “in uncharted waters, where we were replacing doors like we were replacing our underwear.”

“The planes were arriving every day, every day,” the worker added.

“Simple manufacturing” eliminates inspections

A former FAA official told investigators that he attributed the problems to Boeing's move toward a “lean” manufacturing model to try to cut costs, and that, as part of that process, it cut back on inspections.

“Some of the former Toyota managers were hired to build airplanes like Toyota builds cars,” said James Phoenix, a retired FAA administrator who oversaw Boeing. When the FAA asked Boeing to reinstate inspections, he said, “it complied with everything, but very slowly.”

Phoenix told the NTSB that it took two fatal crashes of the 737 Max in 2018 and 2019 for Boeing to give in to FAA demands to restore inspections it had stopped doing.

“That didn’t change until the Max 9 incidents, which highlighted a lot of things. So it takes a lot of effort to get Boeing to change, and when Boeing does change, it’s very slow and takes a long time.” It’s time to understand that their quality system needs to improve.

Lund defended Boeing's use of optimized production, noting that it did not conflict with the goal of safer, better-quality aircraft.

“We truly believe that the main way to improve simplicity is to improve quality,” he said.

While Lund said Boeing is committed to making further improvements, Homendy said the company had plenty of evidence of quality problems and had not done enough to improve its practices until the Alaska Air incident.

“Where do we go in the future? So that we don't end up in another situation… (where new changes are produced) as a reaction to a terrible tragedy,” he said.

CNN's Owen Dahlkamp, ​​Dania Gaynor, Selina Tibor, Nikki Brown, Ramisha Marouf and Samantha DeLuia contributed to this article.