SEATTLE — The fatal flaws with Boeing’s 737 Max can be traced to a breakdown late in the plane’s development, when test pilots, engineers and regulators were left in the dark about a fundamental overhaul to an automated system that would ultimately play a role in two crashes.

A year before the plane was finished, Boeing made the system more aggressive and riskier. While the original version relied on data from at least two types of sensors, the final version used just one, leaving the system without a critical safeguard. In both doomed flights, pilots struggled as a single damaged sensor sent the planes into irrecoverable nose-dives within minutes, killing 346 people and prompting regulators around the world to ground the Max.

But many people involved in building, testing and approving the system, known as MCAS, said they hadn’t fully understood the changes. Current and former employees at Boeing and the Federal Aviation Administration who spoke with The New York Times said they had assumed the system relied on more sensors and would rarely, if ever, activate. Based on those misguided assumptions, many made critical decisions, affecting design, certification and training.

“It doesn’t make any sense,” said a former test pilot who worked on the Max. “I wish I had the full story.”