RENTON, Washington — For months, missing paperwork has hindered the investigation into how a door plug blew off a 737 Max on an Alaska Airlines flight in January, making it difficult to find out who made the near-tragic mistake. This week, Boeing disclosed that the paperwork may have caused the problem in the first place.

It was already well known that no documentation was found to show who worked on the door plug. At a briefing for journalists at Boeing’s 737 Max factory in Renton, Washington, Boeing said that lack of paperwork is why the four bolts needed to hold the door plug in place were never installed before the plane left the factory in October. The workers who needed to reinstall the bolts never had the work order telling them the work needed to be done.

Without the bolts, the door plug incident was pretty much inevitable. Luckily, it wasn’t fatal.

It’s a sign of the problems with the quality of work along the Boeing assembly lines. Those problems have become the focus of multiple federal investigations and whistleblower revelations, and the cause of delays in jet deliveries that are causing headaches for airlines and passengers around the globe.

But Boeing may have landed itself in even more trouble with regulators for divulging the details at this stage. The National Transportation Safety Board (NTSB) reprimanded Boeing Thursday for releasing “non-public investigative information” to the media. It said in a statement that the company had “blatantly violated” the agency’s rules.

“During a media briefing Tuesday about quality improvements ... a Boeing executive provided investigative information and gave an analysis of factual information previously released. Both of these actions are prohibited,” the NTSB said.

Boeing would no longer have access to information generated by the NTSB during its investigation, the agency said, adding it was referring Boeing’s conduct to the Department of Justice.

“As a party to many NTSB investigations over the past decades, few entities know the rules better than Boeing,” the NTSB said.

Boeing did not immediately respond to CNN’s request for comment outside normal business hours.

During the Tuesday briefing, Boeing said that the particular problem with the Alaska Air door plug occurred because two different groups of employees at the plant were charged with doing the work, with one removing and the other reinstalling the door plug as the plane was passing along the assembly line.

The first group of employees removed the door plug to address problems with some rivets that were made by a supplier, Spirit AeroSystems. But they didn’t generate the paperwork indicating they had removed the door plug, along with the four bolts necessary to hold it in place, in order to do that work.

When a different group of employees put the plug back in place, Boeing says the employees didn’t think the plane would actually fly in that condition.

Instead, they were just blocking the hole with the plug to protect the inside of the fuselage from weather as the plane moved outside. That group of employees often makes those kind of temporary fixes.

“The doors team closes up the aircraft before it is moved outside, but it’s not their responsibility to install the pins,” said Elizabeth Lund, senior vice president of quality for Boeing’s commercial airplane unit.

Those employees likely assumed paperwork existed showing that the plug and bolts had been removed, and that paperwork would prompt someone else along the line to install the bolts.

But without the paperwork, no one elsewhere on the assembly line knew that the door plug had ever been removed, or that its bolts were missing, Lund said. Removing a door plug after a plane arrives from Spirit AeroSystems rarely happens, Lund added, so no one was aware the door plug needed attention.

“(Permanent) reinstallation is done by another team based on the paperwork showing what jobs are unfinished,” Lund said. “But there was no paperwork, so nobody knew to follow up.”

The plane actually flew for about two months with the door plug in place despite the lack of bolts. But minutes after the Alaska Airlines flight took off from Portland, Oregon, on January 5, the door plug blew out, leaving a gaping hole in the side of the plane. Passengers’ clothing and phones were ripped away from them and sent hurtling into the night sky. But fortunately no passengers were seriously injured, and the crew was able to land the plane safely.

The missing bolts had been identified in preliminary findings of the National Transportation Safety Board, but that report did not assess blame for the accident. And a final report is not expected for about a year or more. A spokesperson for the NTSB said that the safety agency is continuing its investigation and will not comment on Boeing’s explanation for how the mistake was made.

The board released a preliminary report in February that said it had found the bolts were missing when it left the Boeing factory, but it did not assess blame. A final report is not expected for a year or more from now.

NTSB Chair Jennifer Homendy has testified about the missing paperwork at Congressional hearings since then.

Boeing is addressing the problem by cutting the speed that planes move along assembly lines, and making sure that planes don’t advance with problems under the assumption that those problems will be dealt with later in the assembly process, Lund said.

“We have slowed down our factories to make sure this is under control,” she said.

“I am extremely confident that the actions that we took,” will ensure every airplane leaving this factory is safe, she added. — CNN