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President Donald Trump announced an emergency order from the Federal Aviation Administration on Wednesday grounding Boeing 737 Max jets in the wake of an Ethiopian Airlines crash Sunday and a Lion Air accident in October that together killed 346 people.

Trump's announcement came as the FAA faced mounting pressure from aviation advocates and others to ban flights of the planes pending investigations into the deadly accidents.

Sunday's crash killed 157 people, and the one in Indonesia in October killed 189.

"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," Trump announced, referring to "new information and physical evidence that we've received" in addition to some complaints.

The FAA said it decided to ground the jets after it found that the Ethiopian Airlines aircraft that crashed had a flight pattern very similar to the Lion Air flight.

"It became clear that the track of the Ethiopian flight behaved very similarly to the Lion Air flight," said Steven Gottlieb, the FAA's deputy director of accident investigations.

Meanwhile, the investigation into Sunday's crash was proceeding, with the jet's black boxes to be sent to France on Wednesday night for examination.

U.S. airports and airlines reacted to the order Wednesday, acknowledging that it will lead to canceled flights. Miami International Airport said it expected 19 departures to be canceled Wednesday.

American has about 85 flights a day on Max 8s and Max 9s. United Airlines has about 40 such flights. Southwest Airlines has the most, about 150 a day, out of the airline's total of about 4,100 flights daily.

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The FAA order followed similar directives from a growing number of countries — including Canada, European nations and China — after Sunday's crash.

Boeing Co. said it supported the order and had recommended that the FAA ground its entire global fleet of 371 Max 8 and Max 9 models "out of an abundance of caution and in order to reassure the flying public of the aircraft's safety."

"We are doing everything we can to understand the cause of the accidents in partnership with the investigators, deploy safety enhancements and help ensure this does not happen again," Boeing said.

Sen. Richard Blumenthal, D-Conn., a member of the Commerce, Science and Transportation Committee, said he believed the recent government shutdown delayed the installation of new, enhanced safety software in the Boeing jets after the Indonesia crash.

"The government shutdown absolutely aggravated and exacerbated the failures of the FAA," Blumenthal said, referring to the agency's timetable for installing the software after the accident in October.

"The airline should be held accountable," he said. "They had the new software. They knew of a problem with the sensors" on the jets.

The FAA rebutted the notion that the shutdown delayed the installation of system enhancements.

Congressional Democrats promised an examination of how the FAA previously certified the safety of the Boeing jets.

"There must be a rigorous investigation into why the aircraft, which has critical safety systems that did not exist on prior models, was certified without requiring additional pilot training," House Transportation Committee Chairman Peter DeFazio, D-Ore., and Aviation Subcommittee Chairman Rick Larsen, D-Wash., said in a joint statement. "We plan to conduct rigorous oversight with every tool at our disposal to get to the bottom of the FAA's decision-making process."

Dozens of nations and airlines had grounded the jets before the FAA took action, with the latest being Canada earlier Wednesday.

"This safety notice is effective immediately and will remain in place until further notice," Canadian Transport Minister Marc Garneau said in a statement Wednesday.

The European Union, China and Iraq and airlines like Aeromexico also banned flights of the Max 8 and Max 9 after Sunday's crash, pending safety assurances.

Before the FAA order Wednesday, former Transportation Secretary Ray LaHood said he would be concerned about flying on a Max 8.

"What we need to do is ground the planes, inspect the planes, and use FAA safety inspectors and Boeing safety inspectors collaborating together, figure out if there's something wrong and if there's not, tell the public these planes are safe," LaHood, a Republican who served in the Obama administration, told CNBC about the FAA's earlier decisions to keep the planes flying as other countries grounded them.

Passengers had also been expressing concern, and politicians from both parties had said the planes should be grounded.

Asked whether the planes should be grounded just because people concerned and because of what the FAA had, at one point, called "speculation," LaHood replied: "Well, it's not speculation. I'm suggesting that the agency that has the responsibility for aviation safety step up and carry out their responsibility."

"Talk to the families of the people who went down in the Ethiopian crash," LaHood added. "You talk to those families and you ask them if they wish those planes had been inspected — and the answer will be yes."