Boeing's safety culture and manufacturing quality, both at the center of a corporate crisis following a January mid-air panel blowout on a near-new 737 MAX 9, were scrutinized on Wednesday in two U.S. Senate hearings.

Testimony at the U.S. Senate Permanent Subcommittee on Investigations raised questions about Boeing's treatment of whistleblowers, records surrounding the blown-out door plug on the Alaska Airlines narrowbody jet and production concerns over two separate widebody jets.

Former Boeing engineer Ed Pierson said he turned over records, sent to him from an internal whistleblower, to the FBI that he said provided key information about the Alaska Airlines door plug.

Boeing has said it believed that documents detailing the work on the opening, closing and securing of the door plug that blew out were never created.

The National Transportation Safety Board, which is investigating the blowout, said it "has not received any such documentation from Boeing or any other entity."

The FBI declined to comment.

Since the Alaska Airlines incident on January 5, Boeing has announced a management shakeup, U.S. regulators have put curbs on its production and deliveries fell by half in March.

During the hearing on Wednesday, whistleblower Sam Salehpour, a Boeing quality engineer who raised questions about two of its widebody jets, claimed he was told to "shut up" when he flagged safety concerns. He has said that he was removed from the 787 program and transferred to the 777 jet due to his questions.

Salehpour has claimed Boeing failed to adequately shim, or use a thin piece of material to fill tiny gaps in a manufactured product, an omission that could cause premature fatigue failure over time in some areas of the Boeing 787 Dreamliner. Salehpour said he had reached out to Boeing official Lisa Fahl but was not provided specific safety data.

Fahl has said the 787, which was launched in 2004, had a specification of five-thousandths of an inch gap allowance within a five-inch area, or "the thickness of a human hair."

"When you are operating at 35,000 feet," the size of a human hair can be a matter of life and death, Salehpour told the hearing.

Salehpour's lawyers had previously said documentation he provided to the Federal Aviation Administration (FAA) would be available at the hearing.

The subcommittee's chairman, Senator Richard Blumenthal, held up a 2021 memo from Salehpour and read a line that said "kicking me out of the program because I am raising safety concerns" does not help anybody.

Reuters could not find any documents posted publicly online and was not provided with any by Salehpour's lawyers.

Boeing has challenged Salehpour's claims against the 787 and 777, which are used mostly on international flights, arguing on Monday it has not found fatigue cracks on nearly 700 in-service Dreamliner jets that have gone through heavy maintenance.

In a statement on Wednesday, Boeing defended the planes' safety, noting the global 787 fleet has safely transported more than 850 million passengers, while the 777 has safely flown more than 3.9 billion travelers.

The planemaker added that "retaliation is strictly prohibited at Boeing."

The FAA said in a statement that every aircraft flying is in compliance with the regulator's airworthiness directives.

Earlier in the day, members of the U.S. Senate Commerce Committee said Boeing needs to do more to improve its safety culture, following a February report commissioned after two crashes involving the 737 MAX killed a combined 346 people.

U.S. Senate Commerce Committee Chair Maria Cantwell said she expects Boeing to submit a serious plan in response to a FAA deadline. In late February, the FAA said Boeing must develop a comprehensive plan to address "systemic quality-control issues" within 90 days.