(CNN) — Federal regulators demanded an explanation from Boeing Friday for why the company did not disclose for months “concerning” internal communications relating to the 737 Max certification.

Stephen Dickson, head of the Federal Aviation Administration, requested an “explanation immediately” from Boeing’s CEO “regarding the content of this document and Boeing’s delay in disclosing the document to its safety regulator.”

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The FAA said in a statement accompanying the letter that it has shared the information with congressional committees and the US Department of Transportation inspector general, which is conducting a wide-ranging review of the certification.

The communications are “instant messages between two Boeing employees, characterizing certain communications with the FAA during the original certification of the 737 MAX in 2016,” according to the statement.

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The 737 MAX remains grounded and airlines have canceled flights into next year. Boeing has missed its targets for completing and proving the efficacy of safety fixes to the plane following two crashes that killed 346 people.

Spokesmen for Boeing did not immediately respond to a request for comment.

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