American and Southwest Airlines issued sympathy statements following the second fatal crash of a Boeing 737 MAX aircraft in six months.

An Ethiopian Airlines flight crashed shortly after takeoff from Ethiopia's capital on Sunday morning, killing all 157 on board.

It was not immediately clear what caused the crash of the Boeing 737-8 MAX plane, which was new and had been delivered to the airline in November. The pilot sent out a distress call and was given clearance to return, the airline's CEO told reporters.

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The mishap follows the October 29, 2018 crash of a 2-month-old Lion Air Boeing 737 MAX jet that killed all 189 aboard when the plane plunged into the sea shortly after taking off from Jakarta, Indonesia.

"American Airlines extends our condolences to the families and friends of those on board Ethiopian Airlines flight 302. We will closely monitor the investigation via Boeing and the National Transportation Safety Board,” the airline said. American operates 28 of the Boeing 737 MAX aircraft.

Southwest Airlines issued a similar statement: “Our heart goes out to the families and loved ones of the passengers and Employees on Ethiopian Airlines Flight 302.  As Southwest operates a fleet of 31 Boeing 737 MAX 8 aircraft (as of Dec. 31, 2018), we have been in contact with Boeing and will continue to stay close to the investigation as it progresses.  We remain confident in the safety and airworthiness of our fleet of more than 750 Boeing aircraft. “

Combined, American and Southwest have hundreds of the planes on order.