Two leading House Democrats wrote to the FAA on Thursday demanding to know why the agency appeared to overrule its own engineers’ concerns about safety issues related to the Boeing 737 Max and the 787 Dreamliner, ultimately siding with the manufacturer rather than its own staff.

House Transportation Committee Chairman Rep. Peter A. DeFazio (D-Ore.) and Rep. Rick Larsen (D-Wash.), who chairs the committee’s aviation panel, asked FAA Administrator Stephen Dickson to provide answers about how the agency “weighs the validity of safety issues raised by its own experts compared to the objections raised by the aircraft manufacturers the FAA is supposed to oversee.”

The congressmen wrote: “The two cases … suggest that the opinions and expert advice of the FAA’s safety and technical experts are being circumvented or sidelined while the interests of Boeing are being elevated by FAA senior management.”

The committee has been investigating the development of the Max and its approval by the FAA after the aircraft was involved in a pair of crashes that killed 346 people. The committee learned of the two additional safety issues as part of its investigation, the letter said. They are unrelated to an automated software system that has been implicated in both crashes. Investigations into the crashes have faulted both assumptions Boeing made in designing the automated feature and the FAA’s oversight and safety approval process.

According to the letter, the new issues “both appear to involve serious, potentially catastrophic safety concerns raised by FAA technical specialists that FAA management ultimately overruled after Boeing objected.” The first concerns whether rudder cable protection on the Max is adequate. The second involves lightning protection for fuel tanks on the 787 Dreamliner.

Thursday’s letter comes a week after the committee grilled Boeing chief executive Dennis Muilenburg about the Max, confronting him with documents gathered in its investigation, and indicates that the FAA also will continue to face close scrutiny from the panel.

An FAA spokesman said the agency would respond directly to the congressmen.

Gordon Johndroe, a Boeing spokesman, said both issues raised in the letter were handled properly.

“We are confident that each was properly considered and addressed by Boeing, thoroughly reviewed with and approved by the FAA, and handled in full compliance with the processes governing review and disposition of such issues,” Johndroe said.

The congressmen’s letter describes concerns raised by an FAA manager in 2014 about protections for cables connected to the rudder on the Max. The manager wrote to a more senior FAA official that the protections for the cables were inadequate, which could lead pilots to lose control if the plane’s engines failed.

The FAA initially said Boeing would have to change its design, which the company didn’t want to do, according to the letter. But then in a formal paper addressing the issue, the agency concluded that the design could be left unchanged.

Six FAA specialists objected and a special agency review panel concluded that the FAA should tell Boeing there was not enough evidence to conclude the design complied with safety rules, according to the letter. Nonetheless, the FAA approved the plane as safe soon after.

“It is our understanding that nonconcurrence by FAA technical specialists is fairly infrequent and not to be taken lightly,” the congressmen wrote. “In addition, my staff has been told that it was virtually unprecedented for six or more FAA specialists to jointly non-concur on a single issue, highlighting the gravity of their concerns.”

Regarding the lightning protections on the 787′s fuel tanks, the FAA’s safety office initially rejected changes to the design early this year, only for the agency to reverse its position and approve them.

The congressmen wrote that Boeing had reportedly manufactured 40 airplanes before getting the design change approved by the FAA.

“If accurate, that is an astonishing fact that suggests either willful neglect of the Federal aviation regulatory structure or an oversight system in need of desperate repair,” they wrote.

The FAA’s oversight of Boeing and other aircraft manufacturers has come under close scrutiny after the Max crashes. The agency, under authorization from Congress, has handed over much of the work to determine that Boeing’s planes meet safety standards to the company itself.

Congressional leaders in both the House and the Senate have said in recent weeks they think the system now needs to be changed to restore more of that authority to the FAA.