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.
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.
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.
"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.
The crash of Nairobi, Kenya-bound Ethiopian Airlines Flight 302 shortly after takeoff from the Ethiopian capital was the second deadly crash of a Boeing 737 MAX — one of Boeing's newest and top-selling planes — in less than five months. The same type of plane, operated by Indonesian carrier Lion Air, plunged into the Java Sea minutes after taking off from Jakarta in October, killing all 189 aboard.
United, which has also expressed confidence in its growing fleet of Boeing 737 MAX planes, told Twitter users it operates a larger model of the Boeing 737 MAX, not the model that was involved in the Ethiopian Airlines and Lion Air crashes. It also made a distinction between older and newer Boeing narrow-body jets that have similar model numbers.
Lori Bassani, president of American Airlines' flight attendant union, the Association of Professional Flight Attendants, said she asked the airline's executives on Sunday to "address the critical concerns our members have about flying" the Boeing 737 MAX planes.
Following the Ethiopian crash, airline regulators in China and Indonesia told local carriers Monday to temporarily ground Boeing 737 MAX 8 planes, a move that was followed by Ethiopian Airlines and small carrier Cayman Airways. Late Monday, Delta Air Lines joint venture partner Grupo Aeromexico said it grounded its six Boeing 737 MAX 8 aircraft "until more thorough information" on the crash is provided.
Sen. Dianne Feinstein, D-Calif., asked the FAA to consider grounding the Boeing 737 MAX 8 planes until cause of the crash is determined and deemed safe, citing the similar measures in China and Indonesia.
"I ask that the Federal Aviation Administration evaluate whether similar precautions in the United States are advisable and practicable," Feinstein wrote. "Continuing to fly an airplane that has been involved in two fatal crashes within just six months presents an unnecessary, potentially life-threatening risk to the traveling public."
In its Monday notice stating that it did not have reason to ground the planes, FAA said it expects to mandate design enhancements to the automated system and signaling on board the Boeing planes by no later than April. Boeing is planning to update training requirements and manuals along with those changes, the FAA added.
Boeing shares closed Monday at $400.01, down 5.3 percent, after tumbling as much as 13.5 percent earlier in the day. Southwest fell 0.3 percent, while United was little changed. American Airlines' stock rose 0.4 percent and shares of Delta Air Lines added 3.1 percent.
For its part, Boeing said in a statement it is in contact with both customers and regulators but that it did not see any reason to "new guidance to operators."
In an email to employees, Boeing's CEO Dennis Muilenburg said: "I know this tragedy is especially challenging coming only months after the loss of Lion Air 610. While difficult, I encourage everyone to stay focused on the important work we do. Our customers, business partners and stakeholders depend on us to deliver for them." He urged Boeing staff not to speculate or discuss the accident because doing so "without all the necessary facts is not appropriate and could compromise the integrity of the investigation."
The European Union Aviation Safety Agency on Monday said it's in contact with aircraft manufacturers and its counterpart in the United States, the Federal Aviation Administration. EASA "will immediately decide on actions that may be needed to take at fleet level as soon as the necessary information is available," it said in a statement.
—CNBC's Phil LeBeau contributed to this report.