Video report by ITV News Correspondent Neil Connery
They highlighted poor training and urged for action to be taken immediately during a meeting with a Boeing official after the first deadly 737 Max crash off Indonesia in October.
A Boeing representative is heard resisting their calls but promised a software fix.
But this had not been rolled out when an Ethiopian Airlines' 737 Max crashed four months later, killing 157 people.
Today the mother of one of the victims of the crash laid the blame for her daughter's death on the shoulders of Boeing bosses.
Nadia Milleron lost her daughter Samya Rose Stumo - who was just 24, she told ITV news, ''Boeing owns this''.
In the US, the 737 Max will return to service only when the FAA's analysis of the facts and technical data indicate that it is safe to do so
Mrs Miller told ITV News: ''You think about their future Christmases, maybe she would have gotten married, maybe she would have had children.
''There are huge blanks in our future because we don't have our daughter. Boeing owns this, because they own the defect on the plane whatever they are that we need to discover.
''They also own the lack of training for pilots.''
The US’s top aviation regulator has assured Congress that the Boeing 737 Max, grounded after two deadly accidents, will only return to flying when a government analysis shows that it is safe.
The Federal Aviation Administration is under scrutiny for how it relied on Boeing to certify the Max and then did not ground the plane until after the second crash, in March.
Acting FAA chief Daniel Elwell told the House aviation subcommittee that his agency “welcomes scrutiny that helps make us better”.
Mr Elwell listed several reviews of the FAA’s handling of the matter, adding, however, that only the FAA would decide when the Max is safe enough to allow back in the air.
“In the US, the 737 Max will return to service only when the FAA’s analysis of the facts and technical data indicate that it is safe to do so,” Mr Elwell said.
House Aviation subcommittee chairman Rick Larsen said he expects answers about the FAA’s certification of the Max, the role of Boeing employees in assessing key features on the plane, and FAA’s role in developing pilot training for the plane.
“The FAA has a credibility problem,” he said.
Mr Larsen added a note of economic urgency to the grounding of the Max, Boeing’s best-selling plane and one that is built in his home state of Washington.
He said Congress must help make the public feel safe about flying because “if they don’t fly, airlines don’t need to buy airplanes,” and “then there will be no jobs” in aircraft manufacturing.
Representative Peter DeFazio criticised Boeing for pilot manuals that did not mention a new automated flight-control system implicated in both accidents, and for a design that pitched the plane’s nose down based on readings from a single sensor that could fail.
The two plane crashes in Ethiopia in March and in Indonesia last October killed 346 people.
Mr DeFazio, who heads the full transportation committee, also said Boeing has not yet provided documents that he and Mr Larsen requested, saying he hoped the company would provide them voluntarily and soon.
“Boeing has yet to provide a single document,” he said.
“We’ve got to get to the bottom of this.”
Boeing is already the subject of a criminal investigation by the Justice Department.
Boeing customers Southwest Airlines and American Airlines and their pilot unions have received subpoenas related to that investigation; United Airlines, which also flew the Max until it was grounded in March, declined to comment, although its pilot union confirmed that it too has received a subpoena.
The Transportation Department’s inspector general and a Senate committee are looking into the FAA’s relationship with Boeing, and the House subcommittee is likely to follow a similar path.
The hearing before the House panel is expected to cover the FAA’s review of a flight-control system on the Max that was not present on earlier versions of the 737.
In both accidents, the automated flight system pushed the nose of the plane down and pilots were unable to regain control.
The Dallas Morning News reported that American Airlines pilots pressed Boeing in November, shortly after the first Max crash, on potentially grounding the planes and pushed for a quick software fix from the plane maker.
“We don’t want to do a crappy job of fixing things, and we also don’t want to fix the wrong things,” a Boeing employee responded, according to a recording reviewed by the newspaper.
Mr Elwell was scheduled to be joined at the House hearing by Robert Sumwalt, chairman of the National Transportation Safety Board.
No Boeing representative was scheduled to give evidence.
Meanwhile President Donald Trump’s nominee to head the Federal Aviation Administration said he was committed to making any changes recommended by groups looking into how the agency certifies the safety of airliners.
Under questioning during a Senate committee confirmation hearing, Stephen Dickson said he would never abdicate his responsibility for making sure planes are safe.