The Federal Aviation Administration followed existing rules when it approved the Boeing 737 Max in 2017 but improvements are needed, a panel assembled by the Department of Transportation said Thursday.

The FAA has been under fire for certifying the jets, which were involved in two fatal crashes in 2018 and 2019, prompting the worldwide grounding of the planes in March. The DOT called for the panel after the second crash and the group conducted one of numerous investigations into the plane’s development and certification.

Two House lawmakers last week said they would explore legislation that would give the FAA greater control over the certification process. Under current rules, the FAA handed some certification tasks to Boeing employees.

The DOT’s blue-ribbon panel, which included retired Air Force Gen. Darren McDew, former head of the U.S. Transportation Command, and Lee Moak, former president of the Air Line Pilots Association union, stood by the delegation program to manufacturers. It found, however, that the approval of new jets could be improved by better assessing pilot performance and the “cumulative effect of multiple changes to aircraft design.”

The Boeing 737 Max was an updated version of a plane that has been flying since 1967. Boeing included a flight-control system to avoid stalls but this was later implicated in the two crashes.

Boeing has been scrambling to get the FAA to sign off on its software fixes for the Max, whose grounding has cost airlines more than $1 billion in revenue.

“Safety is a core value at Boeing, and we are committed to continuous improvements in global aerospace safety,” Boeing said in a statement. “We will study these recommendations closely, as we continue to work with government and industry stakeholders to enhance the certification process.”