Boeing Co.’s chief executive officer didn’t provide complete testimony last week on the 737 Max, and congressional investigators are continuing their probe of the troubled jetliner, two House leaders said in a letter to other lawmakers on Monday.

Boeing’s Dennis Muilenburg, who endured hours of grilling by members of the House Transportation & Infrastructure Committee Oct. 30, left “a lot of unanswered questions, and our investigation has a long way to go to get the answers everyone deserves,” said Representative Peter DeFazio, chairman of the committee, and Representative Rick Larsen, leader of its aviation sub-panel.

“Mr. Muilenburg’s answers to our questions were consistent with a culture of concealment and opaqueness and reflected the immense pressure exerted on Boeing employees during the development and production of the 737 Max,” the lawmakers said in the letter.

The letter summarized testimony and documents revealed by the committee, such as a 2015 warning by a Boeing engineer that the system later involved in two fatal crashes wasn’t sufficiently redundant. The committee also raised concerns about whether the design met the company’s own criteria, and if pilots could be expected to handle the emergencies that occurred in the crashes.

“To summarize our key concerns, our investigation shows that from almost the start, Boeing had a bad design on MCAS with a single point of failure,” the lawmakers said in the letter, referring to the system implicated in the crashes.

Muilenburg, who also testified Oct. 29 before the Senate, acknowledged mistakes were made and said the company has taken numerous steps to redesign the 737 Max and to prevent such missteps in the future.

Boeing didn’t immediately respond to a request for comment on the letter.