In the wake of a second deadly crash involving a Boeing 737 MAX 8 Series plane, Boeing's CEO said Monday the company stands by the safety of their products, especially the 737 MAX.

Chairman, president and CEO Dennis Muilenburg wrote in a message to all employees that it "was with a heavy heart" that he learned of the loss of Ethiopian Airlines Flight 302. 

The flight crashed Sunday shortly after takeoff from the Ethiopian capital of Addis Ababa, killing all 157 people aboard.

The crash was similar to that of a Lion Air jet, of the same Boeing model, in Indonesian seas last year, killing 189 people. The crash was likely to renew questions about the 737 Max 8, the newest version of Boeing’s single-aisle airliner, which was first introduced in 1967 and has become the world’s most common passenger jet.

"I know this tragedy is especially challenging coming only months after the loss of Lion Air Flight 610," Muilenburg's statement, obtained by CBS and ABC News, read. "While difficult, I encourage everyone to stay focused on the important work we do." 

Muilenburg explained that the company has delivered more than 370 737 Max airplanes to 47 customers and that the aircraft has completed hundreds of thousands of flights safely since its introduction.

“As we partner with regulators and the Ethiopian Airlines investigation unfolds, it’s necessary that we stay centered on the facts and avoid speculation,” he said.

The message was first tweeted by a reporter from CBS News, who also added that Boeing has been working on design changes mandated by the FAA, like software updates, since the Lion Air crash in October 2018.

Safety experts have cautioned against drawing too many comparisons between the two crashes until more is known. Several international airlines have announced they would be temporarily grounding their MAX 8 fleet.

Ethiopian Airlines decided to ground its remaining four 737 Max 8s until further notice as "an extra safety precaution," spokesman Asrat Begashaw said. The carrier had been using five of the planes and awaiting delivery of 25 more.

Among the airlines still using the plane are Southwest, American and Air Canada.

Meanwhile, U.S. Senator Diane Feinstein sent a letter to the FAA Monday requesting that “all Boeing 737 Max 8 series aircraft be grounded until their safe use has been confirmed.”