Employees work on Boeing 737 MAX airplanes at the Boeing Renton Factory in Renton, Washington on March 27, 2019
Washington CNN  — 

Government officials from civil aviation authorities in nine countries, as well as members of the three US federal agencies, will investigate how the 737 Max airplane was initially certified, according to a release from the Federal Aviation Administration.

The group, called the Joint Authorities Technical Review, will begin work on April 29, and will review the initial certification of “the aircraft’s automated flight control system,” the release states. The review is expected to take 90 days, according to the release. The three American agencies involved in the investigation will be the FAA, National Transportation Safety Board and NASA.

The 737 Max was grounded in March after one of the planes flown by Ethiopian Airlines crashed, killing everyone on board. It was the second accident involving the jet model in less than six months, after another flown by Indonesia’s Lion Air crashed last October. The crashes killed 346 people in total.

The group’s review of the initial certification is separate from the ongoing certification process that Boeing, the company that makes the 737 Max planes, is undergoing with the FAA to re-certify the planes, according to an FAA spokesman.

Boeing has come under intense scrutiny since the Ethiopian airline crash in March. The company at first defended the planes and insisted they were safe to fly, but after intense public pressure, they grounded the planes and admitted that a software fix was needed.

On Wednesday, Boeing said they had made a “significant step” towards FAA certification of the new updated software for the 737 Max. They announced they had completed an engineering test flight with the updated software, which is the key technical flight prior to the certification flight with the FAA.

Earlier this week, American Airlines, the world’s largest airline that owns and operates 737 Max planes, announced they would extend flight cancellations into mid-August because of the Boeing 737 Max grounding.

“Based upon our ongoing work with the Federal Aviation Administration (FAA) and Boeing, we are highly confident that the Max will be recertified prior to this time,” American Chairman and CEO Doug Parker and President Robert Isom said Sunday in a message to airline staff. “But by extending our cancellations through the summer, we can plan more reliably for the peak travel season.”

Approximately 115 flights a day will be canceled through August 19, representing about 1.5% of the airline’s total daily flights, they said.

CNN’s Greg Wallace and Victoria Cavaliere contributed to this report.