Washington (CNN) Federal regulators on Friday demanded Boeing explain why it withheld documentation of employees' concerns with a software system which investigators have linked to two fatal crashes of the 737 MAX.

The instant messages, obtained by CNN from a congressional source, show internal concerns that the MCAS stabilization system was "running rampant" and more powerful than the company had told the Federal Aviation Administration.

"[T]he plane is trimming itself like craxy," one pilot wrote in a message, several of which contain misspellings, later adding, "granted, I suck at flying, but even this was egregious."

He wrote the plane had "some real fundamental issues that they claim they're aware of."

"I basically lied to the regulators (unknowingly)," the pilot wrote.

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