President Donald Trump announced Wednesday that the FAA was ordering the immediate grounding of all Boeing 737 MAX 8 and MAX 9 aircraft in the U.S.
The decision comes after days of mounting pressure from other countries that left the U.S. as one of the last to make the call following a deadly crash in Ethiopia.
"The safety of the American people, of all people, is our paramount concern," Trump said, adding that the grounding would take effect as soon as the planes landed and would continue until further notice. "Pilots have been notified. Airlines have been all notified. Airlines are agreeing with this," he said.
The president said the decision to ground the planes was made based on new evidence that had come to light.
"The FAA is prepared to make an announcement very shortly regarding the new information and physical evidence we've received from the site and from other locations and through a couple of other complaints," he said. "We've had a very, very detailed group of people working on the 737 8 and the 737 9 new airplanes. We're going to be issuing an emergency order of prohibition to ground all flights of the 737 Max 8 and the 737 Max 9 and planes associated with that line."
Shortly afterward, the FAA issued a statement reversing its earlier resistance to grounding the planes, saying "the agency made this decision as a result of the data gathering process and new evidence collected at the site and analyzed today.
"This evidence, together with newly-refined satellite data available to the FAA this morning, led to this decision," the FAA said. "The grounding will remain in effect pending further investigation, including examination of information from the aircraft's flight data recorder and cockpit voice recorder."
In an important caveat, Trump said the decision to ground the planes "didn't have to be made" but was made for a number of reasons, he said, including "psychologically and a lot of other ways."
The president said he'd spoken with the Transportation Secretary Elaine Chao, as well as the CEO of Boeing and the acting administrator of the FAA. All agreed on taking the action, the president said.
In an explanation of how the decision to finally ground the planes was made, a White House official told ABC News' Chief White House Correspondent Jon Karl that up until Wednesday morning "the data was clean" but after Canada shared the satellite data indicating a possible connection, Chao recommended to Trump that the planes be grounded.
It marked the first time Trump directly expressed sympathy for the victims of the crash, calling it a "terrible tragedy."
The president also called Boeing an "incredible company." Just Tuesday, Boeing's CEO had lobbied Trump against any grounding, but the company reversed its stance after Trump's announcement, issuing a statement in support of the move.
"We are supporting this proactive step out of an abundance of caution. Safety is a core value at Boeing for as long as we have been building airplanes; and it always will be," the Boeing statement said.
At the same time, the statement said, "Boeing continues to have full confidence in the safety of the 737 MAX."
Soon after the order, the FAA also said the "black boxes" from the crash -- the flight data and cockpit voice recorders -- would be scheduled for transport to France Wednesday evening for analysis.
The delay in reading out the black boxes, which had yet to be analyzed three days after the crash, played into the decision to ground the Boeing 737 MAX 8 and NAX 9 aircraft, acting FAA Administrator Dan Elwell told ABC News' Chief Transportation Correspondent David Kerley.
As soon as they get them on the table it will help, Elwell said, adding that "our decision process was lengthened more than I would have hoped."
Ethiopia had agreed to send the recordings from Ethiopian Airlines flight 302 abroad for analysis earlier Wednesday, though a destination had not been announced. Officials agreed the country didn't have the technology required to analyze the black boxes, which hold key information on how and why the plane crashed. The boxes are damaged, making data retrieval more difficult.
A key question is whether the plane's advanced flight management system -- the autopilot -- might have played a role in the most recent crash as it did in the fatal crash of an Indonesian Airlines 737 MAX 8 last October. In that crash, it appears the pilots failed to disengage the autopilot when the plane's nose began pitching up and down, perhaps because they were unaware of how to do so.
In the aftermath of that crash, at least two U.S. pilots who have flown the Boeing 737 MAX 8 submitted anonymous reports to NASA, which has an aviation safety reporting system, saying their aircraft suddenly pointed its nose downward after they engaged in autopilot. The accounts were first reported by the Dallas Morning News and obtained by ABC News from NASA.
The pilots disengaged the autopilot, manually corrected the unexpected motion and continued to climb as planned. The flight continued uneventfully after that. It's unclear from the reports if the pilots were both on the same flight or on two separate flights.
In a third report, a pilot reported feeling like the flight manual and training for the Boeing 737 MAX wasn't sufficient.
Any and all details are in high demand by authorities and families of the victims arriving at the crash scene in recent days. Airline customers also increased pressure with overwhelming concerns about flying on the Boeing 737 MAX 8 until the safety of the plane can be better assured.
By Wednesday afternoon, the MAX 8 and MAX 9 were grounded by every country or major airline in the world, including the three airlines operating the aircraft in the U.S.: American Airlines, Southwest Airlines and United, which operates a MAX 9, the longer sibling of the MAX 8.
In addition to Canada's announcement to ground the jets Wednesday, the European Union and the U.K. stopped or banned all Boeing 737 MAX 8 operations on Tuesday, following the decisions of Singapore, Australia, Malaysia, Oman and Ireland to ban the plane from flying in and out of those countries.
As countries and airlines continued to ground the popular plane, flights consistently dropped from the approximately 1,200 Boeing 737 MAX flights per day tracked in the week leading up to the crash on Sunday.
Trump's announcement Wednesday was a marked shift from previous statements from both Boeing and the FAA, which had backed the plane's safety until the FAA emergency order.
Boeing CEO Dennis Muilenburg on Tuesday told the president that the company continued to believe the MAX aircraft was safe, while the Elwell of the FAA maintained that it was extensively reviewing data and would take action if any issues were identified, but had not seen any "systemic performance issues" that would provide basis "to order grounding the aircraft."
ABC News' Matt Gutman and Robert Zepeda contributed to this report from Addis Ababa, Ethiopia. ABC News' Jon Karl, David Kerley and Jeffrey Cook contributed to this report from Washington, D.C.