Shares of Boeing were down 4.2 per cent at $ 329.88 at mid-morning.

Shares of Boeing tumbled again on Monday over its handling of the 737 MAX crisis after US aviation regulators criticized it for withholding key documents for months. Both UBS and Credit Suisse downgraded the company following Friday's statement by the Federal Aviation Administration that called Boeing's handling of the documents "disappointing".

The criticism comes as Boeing's efforts to get the FAA to certify the plane to return to service have dragged on. The jets were grounded in March following two crashes that killed 346 people.

The document is an instant message conversation between Boeing's chief technical pilot for the 737, Mark Forkner, who told a colleague that the performance of Maneuvering Characteristics Augmentation System during a simulation was "egregious" and that he had "basically lied" to regulators inadvertently.

The MCAS is a flight-handling mechanism believed to be at the center of two MAX crashes that killed 346 people, which led to the plane's grounding since mid-March.

Forkner's lawyer, David Gerger, said the conversation concerned the MCAS simulator that "was not reading right" and that Forkner "thought the real plane was safe." Boeing offered a more detailed explanation of the documents on Sunday, saying "we understand and regret the concern" generated by the messages from Forkner, who was involved in developing training and manuals for the MAX.

Boeing said it had not been able to speak to Forkner directly, but pointed to his explanation that his comments concerned a "simulator program that was not functioning properly and was still undergoing testing." The disclosures come ahead of Boeing's earnings release scheduled for October 23 and of a congressional hearing with Chief Executive Dennis Muilenburg at the end of the month.

Earlier this month, Boeing stripped Muilenburg of his title as chairman, sparking speculation that he could soon be ousted from the company.

Shares of Boeing were down 4.2 per cent at $ 329.88 at mid-morning.