Boeing said last week that it had reprogrammed software on its 737 MAX passenger jet to prevent erroneous data from triggering an anti-stall system that is under mounting scrutiny following the two deadly nose-down crashes.
Boeing was expected to complete the work last week, but FAA says the company needs more time to make sure it has identified and addressed all issues.
Acting FAA Administrator Dan Elwell said last week the agency would not unground the planes until its analysis "of the facts and technical data indicate that it is appropriate".
"Boeing's top-selling MAX jet was grounded around the world last month after two fatal crashes involving the model in five months".
Federal prosecutors have asked Boeing to clarify their disclosures on the 737 Max's stall-prevention system.
In Vietnam, Bamboo Airways agreed to buy as many as 26 narrow-body jets from Airbus SE, just a month after saying it was considering ordering as many as 25 Boeing 737 Max planes.
Woods' big run sends him to weekend against McIlroy
The three-time WGC-Match Play victor won the par-5 12th before holing out his second shot from 82 yards at the par-4 13th. McIlroy pounded a drive 395 yards off the tee at 16 while Woods found the lip of a fairway bunker.
A source with knowledge of the investigation has said an anti-stall system, the Maneuvering Characteristics Augmentation System (MCAS), was activated shortly before the crash.
The Boeing 737 Max went down shortly after take-off from Jakarta, killing all 189 people on board.
American civil aviation and Boeing investigators search through the debris at the scene of the Ethiopian Airlines Flight ET 302 plane crash, near the town of Bishoftu, Southeast of Addis Ababa, Ethiopia March 12, 2019.
The pilot had tried repeatedly to regain control and pull the nose up but the plane crashed into the sea.
The same system was implicated in the October crash, with initial investigations finding that a sensor on the plane had transmitted incorrect information to the MCAS system.
According to the document, the European Aviation and Space Agency (EASA) had certified the plane as partly safe because additional training and procedures could "clearly clarify" to pilots on how to deal with unusual situations, in which they might need to deploy a rarely used manual wheel for controlling the angle of the aircraft. Acting FAA chief Daniel Elwell told Senate lawmakers an amended certificate had been given Boeing because the Max 8 and Max 9 were very similar to the company's older 737 models.