Boeing pilots' messages on 737 MAX safety raise new questions

Jaw-dropping allegations involving Boeing and the grounded 737 MAX airplane involved in two deadly crashes.

According to documents seen by Reuters…

Instant messages between the MAX's then-chief technical pilot and another Boeing pilot in 2016 show that the head technical pilot - referring to a key safety system on the 737 MAX -

said that "I basically lied to the regulators unknowingly."

The messages were given to the Federal Aviation Administration by Boeing, subsequently sparking outrage by the head of the FAA, who demanded an "immediate" explanation as to why the information was only recently handed over.

This revelation deepens the crisis for the world's largest planemaker. Its best-selling jet, the 737 MAX, was grounded indefinitely, following a crash of a Lion Air flight in Indonesia last year and an Ethiopian Airlines flight in March. The combined death toll: 346.

Boeing's so-called MCAS anti-stall software has been tied to both fatal crashes.

The messages appear to be the first publicly known observations that MCAS behaved erratically during testing before the aircraft entered service.

Boeing's CEO Dennis Muilenberg is scheduled to testify before Congress for the first time later this month and provide investors with an update next week.

Shares of Boeing tumbled as much as 7 percent on worries this crisis is far from over.