Boeing Chief Executive Dennis Muilenburg was scheduled to appear before the U.S. lawmakers to answer questions following the death of 346 people in both Ethiopian and Lion air crashes months ago.

During the first day of his testimony on Tuesday on the now grounded 737 MAX he faced sharp criticism at a Senate hearing.

U.S. Representative Peter DeFazio, who heads the House Transportation and Infrastructure Committee, said in a written statement that the committee's lengthy investigation into the MAX turned up significant questions about Boeing's decision-making.

"Something went drastically wrong, a total of 346 people died, and we have a duty to fix it," DeFazio will say in his opening statement.

DeFazio said the panel is aware of "at least one case where a Boeing manager implored the then-Vice President and General Manager of the 737 program to shut down the 737 MAX production line because of safety concerns." Boeing did not comment Tuesday on the allegation.

Muilenburg on Tuesday acknowledged errors in failing to give pilots more information on a key safety system known as MCAS before the crashes, as well as for taking months to disclose that it had made optional an alarm that alerts pilots to a mismatch of flight data on the 737 MAX.

"We've made mistakes and we got some things wrong. We’re improving and we're learning," he said.

Taking turns to grill Muilenburg during his first appearance at a hearing on Capitol Hill in the year since the first crash in Indonesia, senators suggested Boeing had not been completely honest and expressed dismay that the 2016 instant messages did not prompt an immediate reaction from the company.

"You have told me half-truths over and over again," Senator Tammy Duckworth told Muilenburg, questioning why the manufacturer did not disclose more details about MCAS's lack of safeguards.

The hearing, the highest-profile congressional scrutiny of commercial aviation safety in years, heaps pressure on a newly rejiggered Boeing senior management team fighting to repair trust with airline customers and passengers shaken by an eight-month safety ban on its 737 MAX following an October 2018 Lion Air crash and the March Ethiopian Airlines crash, which resulted in a combined 346 passenger deaths.

Family members were seated just three rows behind Muilenburg during his testimony and were invited at one point to stand and show large photos of their loved ones.

Nadia Milleron, whose daughter was killed in the Ethiopian Airlines crash in March, said Muilenburg and the board should resign for failing to do more to prevent the crashes.