A Boeing 737 Max 9 plane at the Boeing Field in Seattle, Washington on on March 22, 2019.

The preliminary findings, reported by The Wall Street Journal, are the first to shed light on how the faulty design of the so-called MCAS system, which led to crashes that killed 346 people in October and March, remained in the Max fleet. Boeing's Max jets have been grounded since March.

The Federal Aviation Administration's internal probe of Boeing 's 737 Max approval process has reportedly found that senior agency officials failed to review key safety assessments of the plane's flight-control system that was later implicated in two fatal crashes.

Officials said the results also suggest that during the Max certification, Boeing failed to label the stall-prevention feature as a system in which a malfunction would be disastrous, the Journal reported. That designation would have led to greater examination of the system.

It's still unclear what safety information Boeing disclosed to the FAA during the Max approval process, as well as whether FAA officials conducted their own assessments of the system's safety classification.

The investigation didn't uncover evidence that Boeing intentionally misled the FAA.

Investigators have said that in both accidents, faulty data from a single sensor caused the system to push down the jet's nose into an eventual fatal dive. Boeing has acknowledged that bad data feeding into the MCAS system played a role in the crashes. It plans to submit a software fix to the FAA in upcoming weeks.

The airplane maker has faced intense scrutiny from major airlines, which have have extended Max flight cancellations through the summer.

Southwest has accused Boeing of not disclosing that a standard safety feature designed to warn pilots about malfunctioning sensors had been deactivated on the Max before the Lion Air crash in Indonesia. United said that it learned about the deactivated feature after the Ethiopian Airlines crash in March.

The investigation's findings could be discussed in Wednesday's House Transportation subcommittee hearing. Congress has not yet asked Boeing's CEO to testify about the 737 Max crisis.

Read the report in The Wall Street Journal

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