Boeing President and CEO Dennis Muilenburg admitted the company’s 737 Max planes had design issues as another internal email was made public. The evidence yet again demonstrated employee concerns with the system.

The CEO acknowledged basing the system on a single sensor, called AOA, was one of at least three mistakes in the design of the 737 Max while testifying in front of the House Transportation Committee on Wednesday, CNN reported.

The employee's email revealed concerns about the single sensor system. The message was sent before the two 737 Max planes crashed, which killed 346 people.

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"Are we vulnerable to single AOA sensor failures with the MCAS implementation?" read the email obtained by the news outlet.

Muilenburg also recognized the company’s failure to install a warning light to the system.

"We got that wrong," he said.

The committee criticized Boeing officials who testified for not taking action when another internal document was filed before the crashes, saying the “slow reaction time” to the failure of the MCAS anti-stall system could be “catastrophic,” according to CNN. The MCAS system pushed the planes’ noses down in both fatal crashes.

House Transportation Committee members have looked into both the company after the two fatal crashes and the Federal Aviation Administration’s (FAA) process for approving the plane, including interviewing whistleblowers and reviewing thousands of documents, the network reported. Muilenburg in his testimony called the FAA’s process “a solid system that has been built up over decades” but acknowledged it could use “refinements.”

The CEO also got in a heated exchange with Rep. Steven Cohen (D-Tenn.) when the representative asked him if he was taking a pay cut until the plane’s issues were solved.

"Congressman, it's not about the money for me,” Muilenburg answered, saying the board would decide.

"You're not accountable then,” Cohen answered.

"I am accountable, sir," Muilenburg said.

The CEO also testified in front of the Senate Commerce, Science and Transportation Committee on Tuesday, his first congressional appearance since the crashes, where he faced victims’ families.

The Boeing 737 Max planes were grounded following the Ethiopian Airlines crash this year that killed 157 people. The Lion Air crash, which left 189 people dead, occurred five months prior.