Boeing said Monday that it is suspending production of its troubled 737 Max airplanes next month.

Production of the planes, which were grounded after two crashes killed 346 people, will remain on hiatus until regulators determine when they can be certified and returned to service, Boeing said in a statement.

"Safely returning the 737 MAX to service is our top priority," Boeing said.

The 737 Max was grounded on March 13, three days after an Ethiopian Airlines flight crashed en route to Kenya, killing all 157 people on board.

Another 737 Max, Lion Air Flight 610, had crashed off the coast of Indonesia on Oct. 29, 2018, killing 189 people.

The company said Monday that it continued to build planes after the program was halted and that about 400 are now in storage. Boeing said it would prioritize delivering those planes instead of focusing on more production.

Boeing has said automated anti-stall software contributed to the crashes, but a former Boeing manager recently told NBC News that he alerted the company to problems at its main factory in Washington state in the months before.

The former manager, Ed Pierson, said a push to increase production of the planes had created a "factory in chaos."

"All my internal warning bells are going off," Pierson said in an email to Scott Campbell, the general manager of the 737 Max program, four months before the Lion Air crash. "And for the first time in my life, I'm sorry to say that I'm hesitant about putting my family on a Boeing airplane."

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No evidence links the crashes to the conditions that Pierson, a former Navy squadron commander, said he saw at the plant in Renton, Washington, although his observations have raised new questions about Boeing's hectic push to manufacture the plane.

The company defended its handling of Pierson's concerns, saying they had been scrutinized at the "highest levels" of the company.

"Mr. Pierson raises issues about the production of the 737 MAX, yet none of the authorities investigating these accidents have found that production conditions in the 737 factory contributed in any way to these accidents," the company saidt.