In the days after the first crash of Boeing's 737 Max, engineers at the Federal Aviation Administration came to a troubling realisation: They didn't fully understand the automated system that helped send the plane into a nose-dive, killing everyone on board.

Engineers at the agency scoured their files for information about the system designed to help avoid stalls. They didn't find much. Regulators had never independently assessed the risks of the dangerous software known as MCAS when they approved the plane in 2017.

More than a dozen current and former employees at the FAA and Boeing who spoke with The

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Delegating and deferring

Internal battle at the FAA

Playing down risks