Boeing Co. shares dropped 6.8% on Oct. 18 after the Federal Aviation Administration demanded an immediate explanation regarding a "concerning" and belatedly submitted document related to the grounded 737 Max airplanes.
The FAA said Boeing on Oct. 17 alerted authorities about "instant messages" between two of its employees that depict "certain communications" with the regulator in 2016 when the 737 Max aircraft was being certified.
Reuters first reported, citing documents, that the instant messages suggest Boeing may have misled the FAA about a key safety system on the 737 Max.
"The FAA finds the substance of the document concerning," the regulator said in a statement. "The FAA is also disappointed that Boeing did not bring this document to our attention immediately upon its discovery."
FAA Administrator Steve Dickson said in a letter to Boeing President and CEO Dennis Muilenburg that the company should explain "immediately" the document's content and the delay in its submission.
Boeing said it had discovered the instant messages "some months ago," according to the FAA, which would review the document to determine the appropriate action.
The 737 MAX jets remain grounded globally following two fatal crashes that have been blamed on its anti-stall system.
The FAA maintained that it follows a "thorough process, not a prescribed timeline" for the grounded aircraft's return to service.