Boeing and the Federal Aviation Administration completed their certification test flights on the 737 MAX on Wednesday (July 1), a key milestone toward the plane returning to the sky.

The Federal Aviation Administration announced the news on Wednesday.

The MAX has been grounded since March 2019 after two fatal crashes in five months killed 346 people.

The task now for the FAA is evaluating data from the three days of testing.

In a statement, the FAA said it "is following a deliberate process and will take the time it needs to thoroughly review Boeing's work."

The company is seeking to win approval of the FAA after it made upgrades to the anti-stall system that faulted in both crashes and sent the planes into catastrophic dives.

The 737 MAX fallout has cost Boeing billions of dollars, slashed production and hobbled its supply chain.

Industry and government sources say the FAA will not likely approve the plane's ungrounding until September.

If that happens, the jet would be on track to resume U.S. service before year's end.

Multiple criminal and congressional investigations are still playing out.