The sequence of events, still subject to further evaluation by investigators, calls into question assertions by Boeing and the US Federal Aviation Administration over the past five months that by simply following established procedures to turn off the suspect stall-prevention feature, called MCAS, pilots could overcome a misfire of the system and avoid ending in a crash.

The pilots on Ethiopian Airlines Flight 302 initially reacted to the emergency by shutting off power to electric motors driven by the automated system, these people said, but then appear to have re-engaged the system to cope with a persistent steep nose-down angle. It wasn’t immediately clear why the pilots turned the automated system back on instead of continuing to follow Boeing’s standard emergency checklist, but government and industry officials said the likely reason would have been because manual controls to raise the nose didn’t achieve the desired results.

After first cranking a manual wheel in the cockpit that controls the same movable surfaces on the plane’s tail that MCAS had affected, the pilots turned electric power back on, one of these people said. They began to use electric switches to try to raise the plane’s nose, according to these people. But the electric power also reactivated MCAS, allowing it to continue its strong downward commands, the people said.

The same automated system, also implicated in a 737 MAX crash in Indonesia in late October, has become the focus of various congressional and federal investigations, including a Justice Department criminal probe.

The latest details are based on data downloaded from the plane’s black-box recorders, these people said. They come as Ethiopian investigators prepare to release their report about their preliminary conclusions from the accident, anticipated in the coming days.

Investigators probing the Oct. 29 crash of Lion Air Flight 610 believe erroneous data from a single sensor caused the MCAS system to misfire, ultimately sending the plane into a fatal nose-dive and killing all 189 people on board. Some of the same key factors were at play in the Ethiopian crash, according to people briefed on the details of both crashes.

After the Lion Air accident, Boeing and the FAA issued bulletins to 737 MAX operators around the world reminding them of the existing procedure pilots are trained to follow should the plane’s flight-control system go haywire and mistakenly push down the nose. Those are the steps the Ethiopian pilots initially took months later, these people said.

That procedure works to disable the new MCAS, much like another flight-control feature on earlier 737 models, by cutting power. The plane maker and FAA’s bulletins highlighting that safeguard were often mentioned after the Lion Air accident when U.S. aviation industry officials vouched for the aircraft’s safety.

Boeing Chief Executive Dennis Muilenburg noted the procedure in a Nov. 13 television interview when asked about information given to pilots.

“In fact, that’s part of the training manual,” Mr. Muilenburg said on Fox Business Network, adding the manufacturer was confident in the plane’s safety. “It’s an existing procedure so the bulletin we put out…pointed to that existing flight procedure.”

At a briefing for reporters last week, a Boeing official noted investigations of both crashes were continuing but didn’t comment about specifics when he outlined a coming software fix for the MCAS system and related training changes.

The revised system will rely on two sensors, instead of one as originally designed, to prevent erroneous data triggering it. The system will now be designed to make it less aggressive and allow pilots more control over it, according to previous Boeing and FAA statements.

Mike Sinnett, Boeing’s vice president of product strategy, said last week the plane maker had “complete confidence that the changes we’re making would address any of these accidents.”

The software fix could come as soon as mid-April, according to a person briefed on that issue, but further tests are needed before regulators can approve and mandate it so the grounded fleet can return to service. Another person close to the process, however, said final FAA reviews and tests could take up to six weeks. After that, it could take months longer for some overseas regulators to review and certify the fix for aircraft they oversee.

Activation of MCAS and a related pilot alert, which warns pilots of an impending aerodynamic stall, had been reported previously regarding the Ethiopian crash. But in the wake of the tragedy, Boeing, the FAA and Ethiopian authorities leading the probe have refrained from making any comments about whether the crew followed Boeing-sanctioned procedures to cope with the emergency.

Going forward, aviation experts, regulators and pilots debating the relevant safety issues will have to consider the implications that while the pilots did take such steps in the beginning, those apparently didn’t work as expected likely due to the plane’s speed, altitude and other factors. Eventually, the crew veered to other, nonstandard procedures that made their predicament even worse.

Another issue likely to be raised by the preliminary Ethiopian report is why a single sensor malfunctioned or somehow may have been damaged shortly after takeoff – touching off the deadly chain of events.

Write to Andy Pasztor at [email protected] and Andrew Tangel at [email protected]