A possible clue found in the crash of the Boeing 737 MAX 8 in Ethiopia.
Sources have told Reuters that investigators at the crash site near Addis Ababa have found a key part of the plane's tail.
The horizontal stabiliser is reportedly set in an unusual position - similar to that seen in the wreckage of an Indonesian MAX 8 which crashed last year.
The position of the stabilizer could help determine whether the plane was set nose down for a steep dive.
All Boeing MAX jets now grounded worldwide because of similarities between the two crashes.
Problems with the plane's software suspected in the Indonesian incident.
Before the jets are allowed back in the air, Boeing and U.S. regulators will have to agree a fix.
But that's just the first step, says Reuters correspondent David Shepardson.
(SOUNDBITE) (ENGLISH) DAVID SHEPARDSON, REUTERS WASHINGTON CORRESPONDENT SAYING: "Then it has to be tested, approved and then training procedures have to be rolled out and it has to be actually implemented across the fleet.
So in a minimum, just finalizing the software upgrade will take weeks.
Lawmakers told us today that it would not be on all planes at least until the end of April, potentially longer.
Yesterday the FAA Administrator said the software upgrade could take a couple of months." And that's only if the investigation doesn't turn up any more problems. Black boxes from the Ethiopian jet are now being analyzed in France.
Initial conclusions expected in a few days.
In the meantime, Boeing will keep making its best seller, but deliveries to airlines are suspended.
Analysts estimate that could cost Boeing up to $2.5 billion a month in delayed payments.
And that doesn't include any reimbursement to airlines for flight disruptions.
Meanwhile relatives of the dead stormed out of a meeting with Ethiopian Airlines on Thursday (March 14), decrying a lack of transparency, while others made the painful trip to the crash scene.