NEW YORK — Air safety regulators successfully completed three days of flight tests on the Boeing 737 MAX, a key step in re-certifying the plane, US officials said Wednesday.

While the flight tests in Seattle are "an important milestone ... a number of key tasks remain, including evaluating the data gathered during these flights," the Federal Aviation Administration said Wednesday.

"We will lift the grounding order only after FAA safety experts are satisfied that the aircraft meets certification standards."

The MAX has been grounded worldwide since March 13, 2019, following an Ethiopian Airlines crash that killed 157 people. That catastrophe came just a few months after a Lion Air MAX crash that killed 189 people.

Besides evaluating data from the test flights, regulators still must develop pilot training protocols for the MAX that will be subject to public comment and a final review by a technical advisory board.

All MAX aircraft manufactured since the crashes also will need to be personally inspected by FAA staff, the agency said.

The MAX's anti-stall flight system, the MCAS, was partially to blame for both crashes. But other technical malfunctions, including one involving electrical wiring, were subsequently detected during the aircraft's modification process, slowing down its re-certification

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