Boeing's newest 737 model, the MAX, was grounded worldwide in March following two fatal crashes, one on Lion Air in Indonesia in October and another on Ethiopian Airlines in March, which together killed all 346 on board.
Chief Executive Officer Dennis Muilenburg said on Friday said the company now knows that a chain of events caused both disasters, with erroneous activation of so-called MCAS anti-stall software "a common link" between the two.
Three times the captain, Yared Getachew, cried "pull up", before the Boeing Co plane plunged into a field six minutes after takeoff from Addis Ababa, killing all 157 passengers and crew, said the report by Ethiopian investigators.
The amount and quality of training that Boeing and airlines provided to 737 MAX pilots is one of the issues under scrutiny as investigators around the world try to determine the causes of two 737 MAX crashes within five months.
A Boeing official in Seattle said on Wednesday the timing of the software upgrade was "100 per cent independent of the timing of the Ethiopian accident," and the company was taking steps to make the anti-stall system "more robust."