(CNN) — Boeing’s flawed design of a stabilization system involved in two fatal crashes was among the company’s mistakes in the 737 MAX, CEO Dennis Muilenburg told Congress on Wednesday, as another internal email was made public showing an employee had concerns with the system well before the two crashes.

Muilenburg identified basing the system on a single sensor — known by the acronym AOA sensor — as one of at least three mistakes made during design and production of the plane that remains grounded.

The House Transportation Committee asked about a newly released internal email discussion between Boeing employees in 2015 — more than a year before the plane’s final approval for flight — raising concerns about that design.

“Are we vulnerable to single AOA sensor failures with the MCAS implementation,” the employee, whose name is redacted in the document, asked, raising the exact scenario blamed for the two crashes.

In addition to communication shortcomings, Muilenburg also cited the company’s failure to properly connect a warning light related to the system as a mistake.

“We got that wrong,” he said.

Boeing chief engineer John Hamilton told the committee that single points of failure are allowed for some features, and was permitted for the MCAS anti-stall system.

But committee members raised the prospect the company should have initially made a more stringent design. They pointed to another internal document, dated prior to the crashes, noting that “a slow reaction time” to a MCAS malfunction “found the failure to be catastrophic.” In this document, “slow” was defined as longer than 10 seconds.

The House Transportation Committee has been investigating the two Boeing crashes, which together killed 346 people, and is also scrutinizing the Federal Aviation Administration’s procedures for certifying aircraft.

Muilenburg said he believes the FAA’s approach is “a solid system that has been built up over decades.”

“I think we’ve identified a couple of areas, we could look at refinements,” he said, including “long-standing” assumptions about how pilots react to emergencies.

It has interviewed whistleblowers and reviewed hundreds of thousands of pages of documents, Transportation Chairman Peter DeFazio told reporters this week.

The documents show a “pattern of extraordinary production pressure” to keep up with a new jet from its competitor, Airbus, DeFazio said. A culture of pressure on employees came from very high levels within Boeing and “permeated the organization.”

Fiery exchange over Muilenburg’s pay

Muilenburg avoided discussing details of his compensation when pressed by Rep. Steve Cohen, a Democrat from Tennessee.

“Are you taking a cut in pay, are you working for free from now on, until you can cure this problem?” Cohen asked.

“Congressman, it’s not about the money for me,” Muilenburg said. He said the company’s “board will make those determinations.”

Cohen fired back: “You’re not accountable then.”

“I am accountable, sir,” Muilenburg replied.

Rep. Paul Mitchell, a Republican from Michigan, said he does not “care about bonuses,” but asked if Muilenburg had offered his resignation.

Muilenburg said he has not offered to resign, but acknowledged that the crashes “happened on my watch.”

Muilenburg’s appearance follows a hearing with a Senate committee on Tuesday. In both, he apologized to the victims’ family members, who were in the audience.

Boeing says it anticipates the plane returns to commercial flight before the end of the year, and the grounding weighs heavily on its bottom line. Muilenburg said Tuesday the work is “in the final stages” and he believes the plane will be re-approved for commercial flight in the “next weeks and months.”

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