(Corrections & Clarifications: This story has been updated to clarify the vacation travel of Melvin and Bennett Riffel.)
Days after an Ethiopian Airlines flight plowed into the ground after takeoff from Addis Ababa, killing all 157 people aboard, international investigators are still searching for answers.
Officials know the 4-month-old Boeing 737 MAX 8 plane crashed six minutes into its flight to Nairobi on Sunday morning after its pilot, who had more than 8,000 hours of flight experience, issued a distress call. The airline said crews did a "rigorous" maintenance check of the plane on Feb. 4.
A team of National Safety Transportation Safety Board and Federal Aviation Administration investigators arrived at the crash site early Tuesday to aid the crash probe.
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The tragedy comes less than five months after a Lion Air plane of the same model crashed into the Java Sea 12 minutes after departing from Jakarta, Indonesia, killing all 189 passengers and crew.
Debuting in 2017, 74 MAX 8's fly in the U.S. and 387 fly worldwide. China, Indonesia, Singapore and Australia have temporarily grounded the planes, along with numerous airlines around the world.
In the U.S., the FAA has ordered no such grounding but said it expects to require Boeing to complete flight control system enhancements for the plane by month's end.
The stakes for Boeing are high: Airlines have ordered 4,661 more of the planes – the newest version of the 737 and best-selling airliner ever.
Here are three unanswered questions surrounding Sunday's crash:
Why did the plane crash?
We'll know soon since officials have recovered the black box voice and data recorders.
“They already have the black boxes and they will be able to read them in the next 48 hours,” said Michael Barr, an aviation safety expert who instructs at the University of Southern California’s Viterbi School of Engineering. “It’s going to be pretty quick.”
In the Lion Air investigation, it took officials one month to announce the plane was not airworthy on a flight the day before it crashed. Investigators determined an automatic safety feature repeatedly pulled the plane's nose down as the pilots struggled to control the plane.
The Air Line Pilots Association, which represents more than 61,000 pilots at 33 airlines, warned against jumping to conclusions.
“As the various parties responsible for this investigation begin their work, we caution against speculation about what may have caused this tragic accident,” the organization said in a statement. “ALPA stands ready, through the International Federation of Air Line Pilots’ Associations, to assist the international aviation community in every way possible with the shared goal of advancing a safer air transportation system around the globe.”
Will the U.S. ground the plane model?
Some experts and legislators have called upon regulators to ground all MAX 8 planes as a precaution, but airlines operating the model expressed confidence Monday in their fleets.
Southwest Airlines, which operates 34 of the planes, said on Twitter that the airline had completed 31,000 flights using the model and planned to move forward with them. American Airlines, which has 22 of the planes, doesn't plan to ground them, either, even as the union representing its flight attendants issued a bulletin detailing its concerns.
Boeing 737 Max: What you should know if you're booked on a flight
U.S. senators Dianne Feinstein, D-California, and Richard Blumenthal, D-Connecticut, called on the Federal Aviation Administration and the airline industry to take precautionary measures.
“Until the cause of the crash is known and it’s clear that similar risks aren’t present in the domestic fleet, I believe all Boeing 737 MAX 8 series aircraft operating in the United States should be temporarily grounded,” Feinstein wrote. “This aircraft model represents only a small fraction of the domestic fleet, and several other countries have already taken this important step, including China and Indonesia.”
Mexican airline Aeromexico and Brazil’s Gol Airlines announced Monday night that they would suspend use of MAX 8 airplanes. And, early Tuesday, India’s Jet Airways announced it was grounding its five MAX 8 planes.
U.S. Secretary of Transportation Elaine Chao said the department is monitoring the investigation, stressing that "safety is our first priority."
"I want travelers to be assured and that we are taking this seriously and monitoring latest developments," Chao said in a statement.
Who are the victims?
Investigators are still identifying the victims, including 149 passengers and eight crew members. The dead include at least 21 United Nations staff members along with others who worked with the organization, according to U.N. Secretary-General Antonio Guterres.
Among the eight Americans who died were brothers from California, Melvin and Bennett Riffel. The Redding residents were traveling to Africa as part of a vacation.
Jake Mangas, a friend of the Riffel family, said Melvin was expecting to become a father in the spring.
"Our family is devastated for Ike and Susan (the brothers' parents) and certainly for Melvin's wife, Brittney," Mangas said. "They are wonderful, faith-filled people and if there is any encouragement to me, it's in this difficult circumstance, I know they are surrounded by a community that loves them very much."
A third-year Georgetown Law student was also lost. Cedric Asiavugwa was heading to Nairobi after his fiance's mother died, according to the university.
Contributing: John Bacon, USA TODAY; David Benda, Redding Record Searchlight; The Associated Press
This article originally appeared on USA TODAY: Will the US ground Boeing 737 Max 8 jets? Questions following Ethiopian Airlines crash left unanswered