U.S. investigators on Thursday sanctioned Boeing for revealing details of a probe into a 737 MAX mid-air blowout and said they would refer its conduct to the Justice Department, prompting the embattled planemaker to issue an apology.

The U.S. National Transportation Safety Board said Boeing had "blatantly violated" its rules by providing "non-public investigative information" and speculating about possible causes of the Jan. 5 Alaska Airlines (ALK.N), opens new tab door-plug emergency during a factory tour attended by dozens of journalists.

The decision sheds new light on strains between the crisis-hit planemaker and government agencies at a time when it is trying to avoid criminal charges being weighed by the Department of Justice (DOJ) ahead of a July 7 deadline.

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

The NTSB said Boeing would keep its status as a party to the investigation into the Jan. 5 Alaska Airlines emergency but would no longer see information produced during its probe into the accident, which involved the mid-air blowout of a door plug with four missing bolts.

Unlike other parties, Boeing will now not be allowed to ask questions of other participants at a hearing on August 6-7.

"We deeply regret that some of our comments, intended to make clear our responsibility in the accident and explain the actions we are taking, overstepped the NTSB’s role as the source of investigative information," Boeing said in a statement.

The NTSB's criticism revolves around comments made during a media briefing about quality improvements on Tuesday at the 737 factory near Seattle - widely seen as part of an exercise to showcase greater transparency ahead of the Farnborough Airshow.

During the briefing, which was held on Tuesday under an embargo allowing contents to be published on Thursday, an executive said the plug had been opened on the assembly line without the correct paperwork to fix a quality issue with surrounding rivets, and that missing bolts were not replaced.

The team that came in and closed the plug was not responsible for reinstalling the bolts, Elizabeth Lund, Boeing's senior vice president of quality, added.

The NTSB said that by providing investigative information and giving an analysis of information already released, Boeing had contravened its agreement with the agency.

"Boeing offered opinions and analysis on factors it suggested were causal to the accident," it added.

DOJ SCRUTINY

In May, the DOJ said Boeing had violated a 2021 settlement with prosecutors that shielded it from criminal charges over interactions with the Federal Aviation Administration prior to MAX crashes in 2018 and 2019 that killed 346 people.

U.S. prosecutors have recommended criminal charges be brought, Reuters reported on Sunday. The DOJ already has a separate criminal probe into the door-plug episode.

Thursday's rare exchange marks the latest sign of strains between Boeing and the NTSB.

In 2018, Boeing was widely criticised for issuing a statement appearing to question the performance of pilots in the first of two fatal crashes that led to a grounding of the MAX. Later investigations emphasised the role of flawed software.

In March this year, NTSB Chair Jennifer Homendy told a U.S. Senate hearing Boeing had failed to provide names of employees on its 737 MAX door team for two months, drawing criticism from lawmakers. Boeing then quickly provided the names.

On Thursday, the NTSB said Boeing had portrayed its investigation of the Alaska air incident to media as a search to locate the individual responsible for plug work.

"The NTSB is instead focused on the probable cause of the accident, not placing blame on any individual or assessing liability," the agency said.

Asked during Tuesday's briefing who had failed to fill in documentation, Lund said: "There may have been one or more than one employee. What I will say is the 'who' is absolutely in the responsibility of the NTSB. That investigation is still going on and I am going to not comment on that right now".

The role of individuals is a particularly sensitive topic in air safety amid an increasing focus on litigation and, in some countries, a trend towards criminalising air accidents.

Under global rules, agencies carry out civil probes into air accidents for the sole purpose of finding the cause and making recommendations to improve safety in future. Such actions are separate from any judicial probes seeking to attribute blame.

Aviation experts say an 80-year-old international treaty that encourages people to speak freely and focus on causes rather than blame allowed the industry to cut the number of accidents dramatically since the start of the jet age, but depends on curbing any special pleading by the parties involved.

Critics, including some lawyers, say this system does not sufficiently take account of the need of the families of victims for detailed answers.

In 2013, the NTSB barred United Parcel Service (UPS.N), opens new tab and its pilots union from an investigation of a crash in Alabama that killed two UPS pilots.

In 2018, it removed Tesla as a party to an investigation into a fatal crash involving a vehicle's "Autopilot" system. Tesla hit back publicly, saying it had already decided to withdraw and accusing the NTSB of violating its own rules.