Congressional leaders who oversaw an 18-month investigation into design and oversight problems with the Boeing 737 Max on Thursday called on the Federal Aviation Administration to publicly release the documentation and data the agency is using to determine whether the plane is safe to fly again.

Two 737 Max jets crashed in a five-month span, killing 346 people, and the plane has been grounded since March 2019. FAA administrator Steve Dickson said Wednesday after flying the updated plane that the agency is in the “home stretch” in its safety evaluations. Boeing said it made comprehensive fixes to the plane’s software.

In their letter, Rep. Peter A. DeFazio (D-Ore.), chair of the House Transportation Committee, and Rep. Rick Larsen (D-Wash.) called on the FAA “to publicly release all documents related to design revisions or evaluations related to the aircraft’s safe return to service.”

Michael Stumo, whose daughter Samya was killed on the Ethiopian Airlines flight that crashed on March 10, 2019, had made a similar request Wednesday and Dickson said at a news conference that he faced limits in revealing such technical details.

“We’re providing everything we can, within the law. Much of the data, I believe, that’s being asked for is proprietary,” Dickson said Wednesday.

But DeFazio and Larsen pressed the issue again Thursday “in the strongest possible terms.” Both Boeing and the FAA had mistakenly found the flawed flight control feature that led to the crashes “to be compliant” with federal safety standards, according to their letter, “despite the fact that the aircraft was actually unsafe.”

“To assure the flying public that Boeing’s fixes to the MAX have rendered the plane safe to once again carry passengers, the FAA will need to do more than merely certify that the plane is now compliant,” they wrote.

They said Dickson should release, among other documentation, “system safety assessments, related analysis, assumptions about pilot response times and key test data concerning the safety of the aircraft.” They said the FAA “should fully reveal the data any determination to unground the MAX has been based upon.”

In a statement, the FAA declined to say whether it would release the requested data or call on Boeing to release such information itself.

“We will respond directly to the members,” the statement said.

Boeing declined to address whether it supports the release of the information or would agree to waive any claim that the requested documentation may be proprietary.