Boeing CEO Dave Calhoun met on Wednesday with several U.S. lawmakers on Capitol Hill, but two senators said he provided no answer about what caused the mid-air cabin panel blowout on a 737 MAX 9 this month and that congressional oversight will be needed.

“The American flying public and Boeing line workers deserve a culture of leadership at Boeing that puts safety ahead of profits,” Senator Maria Cantwell said in a statement after meeting with Calhoun. A spokesperson for Cantwell said she was meeting with Calhoun at Boeing’s request.

The Senate Commerce Committee, which Cantwell chairs, will hold hearings to investigate the root causes of the safety lapse, she said.

Asked about whether Boeing installed the plug door that blew out on the Jan. 5 Alaska Airlines flight, Senator Jerry Moran, the top Republican on the Commerce aviation safety subcommittee, told reporters in Washington that Boeing “generally understand(s)” the cause of the Jan. 5 blowout on an Alaska Airlines plane but is waiting for the National Transportation Safety Board (NTSB) to finalize its investigation.

“I did not learn what they believe that to be, but they did convince me that they are seriously trying to figure out the answer to that,” he said. Moran represents Kansas, where Spirit AeroSystems makes the door plug that was torn off the MAX 9 in midair.

Boeing declined to comment on the CEO’s series of engagements on Capitol Hill, but Calhoun told reporters before one meeting that Boeing fully understands “the gravity of the situation.”

“We don’t put planes in the air that we don’t have 100 percent confidence in,” Calhoun said.

Calhoun’s visits with lawmakers came as the NSTB announced plans to travel to Boeing’s 737 factory in Renton, Washington, on Friday.

The NTSB said it will examine documents on the MAX 9 plane that suffered the blowout, including those related to production, manufacturing and maintenance, allowing the organization to “build a timeline from the early stages of production of the door plug to the accident flight.”

The U.S. Federal Aviation Administration (FAA) grounded most of those jets for checks after a plug replacing an unused exit door tore off an Alaska Airlines plane on Jan. 5, forcing an emergency landing.

Calhoun said senators had a lot of questions and vowed to be as transparent as possible in discussing the Alaska Airlines incident.

He referred a question about a Seattle Times report that Boeing had removed a component called a door plug that blew out on that Alaska Airlines MAX 9 to the National Transportation Safety Board, which did not comment.

“We believe in our airplanes,” Calhoun said. “We have confidence in the safety of our airplanes.”

Moran said Calhoun indicated during the meeting that, no matter the root cause of the accident, “Boeing is responsible because at the end of the day, they’re the ones who are signing off and selling the airplane to a customer,”

FAA Administrator Mike Whitaker told Reuters this month that the agency was “going through a process to work out how to restore confidence in the integrity of these plug doors.”

Cantwell and Senator Ted Cruz, the committee’s top Republican, held a closed-door briefing last week on the grounding with FAA Administrator Mike Whitaker and NTSB Chair Jennifer Homendy. Cruz will meet with Calhoun on Thursday.

Alaska Airlines CEO Ben Minicucci told NBC News the airline found “some loose bolts on many” Boeing 737 MAX 9 jets during inspections. The FAA is still reviewing data from an initial group of 40 planes.

(Reporting by David Shepardson and Valerie Insinna; Editing by Chizu Nomiyama, Paul Simao and David Gregorio)