US lawmakers have accused Boeing of engaging in a "pattern of deliberate concealment" as it sought approval for its 737 Max 8 plane to fly.
The accusation came as Boeing chief executive Dennis Muilenburg was quizzed by the Senate Commerce Committee, BBC reports.
Senators said they had serious concerns that Boeing put profits over safety as it rushed to get clearance.
Two deadly 737 Max 8 crashes killed a total of 346 people. Mr Muilenburg admitted the firm had made "mistakes".
"We have learned from both accidents and identified changes that need to be made," he said.
In October last year, a Boeing 737 Max operated by Lion Air crashed, killing all 189 people on board.
Five months later an Ethiopian Airlines plane crashed, killing 157, after which the entire 737 Max fleet was grounded.
Lawmakers accused Boeing of being aware of problems in the automated control system in the 737 Max 8, known as MCAS, which has been identified as a factor in both accidents.
Senator Roger Wicker said messages between Boeing staff that discussed problems in the MCAS test system betrayed "a disturbing level of casualness and flippancy".
Senator Richard Blumenthal said Boeing had engaged in a "pattern of deliberate concealment".
Boeing provided the messages to the committee ahead of the testimony.
Boeing CEO Muilenburg acknowledged errors in failing to give pilots more information on MCAS before the crashes in Ethiopia and Indonesia, as well as for failing to tell the Federal Aviation Administration for months that it made an alarm alerting pilots to a mismatch of flight data optional on the 737 MAX, reports Reuters.
"We've made mistakes and we got some things wrong. We're improving and we're learning," he said.
For months, Boeing had largely failed to acknowledge blame, instead vowing to make a "safe plane safer." Tuesday's hearing represents Boeing's broadest acceptance of responsibility that it made mistakes.