U.S. airlines tried to assure nervous customers Monday that the Boeing 737 MAX jets they fly are safe, a day after one of the new jets operated by Ethiopian Airlines crashed outside of Addis Ababa and killed all 157 people on board.

Accident investigators from Ethiopia and the U.S. are looking for clues as to what brought down the flight and analysts have cautioned that it's too early to know the cause. Investigators have recovered the so-called black boxes, which contain data showing the flight's movements and cockpit voice recordings.

U.S. federal aviation officials on Monday said they still consider the Boeing 737 MAX planes airworthy.

"External reports are drawing similarities between this accident and the Lion Air Flight 610 accident on October 29, 2018," the Federal Aviation Administration said in its notice. "However, this investigation has just begun and to date we have not been provided data to draw any conclusions or take any actions."

The crash has sparked concern among some lawmakers, flight attendants and members of the public, who asked airlines on social media whether these planes are safe and in some cases, whether they can switch their flights.

"We have not relaxed our fare rules or restrictions at this point," said Southwest Airlines spokesman Chris Mainz. Southwest had 34 of Boeing 737 MAX 8 planes in its fleet of about 750 as of the end of last year and remains "confident in the safety and airworthiness" of its aircraft, the carrier said in a statement.

Southwest doesn't charge flight change fees like other airlines but passengers flying on different days and flights will have to pay a difference in fare.

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American Airlines issued a similar statement and said it had full confidence in its planes and crew members. The airline has 14 of the Boeing 737 MAX 8s in its fleet and has not altered its ticket change polices as of Monday morning.

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Some cabin crew members have expressed concerns about the crash.

The Association of Flight Attendants-CWA, which represents 50,000 flight attendants at 20 airlines including United, said it was formally requesting that the Federal Aviation Administration investigate the plane. United operates a larger model of the Boeing 737 MAX.

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"While it is important that we not draw conclusions without all of the facts, in the wake of a second accident, regulators, manufacturers and airlines must take steps to address concerns immediately," said the AFA's international president, Sara Nelson.