WASHINGTON, Oct. 18 (Xinhua) -- The U.S. Federal Aviation Administration (FAA) said Friday that Boeing covered up messages made by its two employees in 2016 about the problematic flight-control system implicated in two deadly crashes of 737 Max planes.

It came after Boeing alerted the U.S. Department of Transportation on Thursday about the existence of those instant messages, several months after the company discovered the communications.

A Boeing pilot told his co-workers the flight handling system was "running rampant" during simulator tests. According to the transcript obtained by NBC News, the pilot said "this was egregious," but "I basically lied to regulators (unknowingly)."

The FAA said in a statement released on Friday that it was "disappointed that Boeing did not bring this document to our attention immediately upon its discovery." The U.S. airplane regulator certified the 737 Max planes in 2017.

A Lion Air Boeing 737 Max 8 plane crashed in October in 2018 in Indonesia, killing all 189 people on board. Later in March, an Ethiopian Airlines Boeing 737 Max 8 plane also crashed en route from Addis Ababa to Nairobi, Kenya, killing all 157 people on board.

FAA chief Steve Dickson demanded Boeing to give an explanation on the content of this document and its delay in disclosure.

An international panel of safety regulators reported last week that the FAA relied too much on Boeing employee for 737 Max's safety and was less capable of effectively certifying the planes.

The multi-agency report said if the FAA technical staff had been fully aware of the details of the flight control system, the agency would probably have required additional scrutiny of the system that might have identified its flaws, thus avoiding accidents.