A former Boeing employee raised concerns about production problems with the Boeing 737 Max months before the two plane crashes in Indonesia and Ethiopia killed 346 people. According to The New York Times, Ed Pierson—a ex-senior manager at a production facility who will testify to Congress on Wednesday—said he witnessed troubling incidents during the production process: tired employees making mistakes, workers pressured to toil faster to meet deadlines, tools going missing, parts getting damaged, and more. He believes these production issues have a direct link to the two plane crashes and 13 other safety incidents that occurred on new 737 Max planes.

In a June 2018 email to the 737 program’s head, Pierson wrote that all of his “internal warning bells” were going off. “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,” he wrote. He retired in August, but said he directly contacted Boeing’s CEO after the Lion Air crash in October 2018. Pierson said he spoke with Boeing’s lawyers, but the company did nothing in response. He also said he wrote to Boeing’s board in February 2019, but did not receive a response from them even after an Ethiopia Airlines plane crashed in March.

According to Pierson, his efforts to contact the Federal Aviation Administration, the National Transportation Safety Board, and the Transportation Department have yielded disappointing results. He has since retained a whistleblower attorney, and is slated to testify before a House Transportation Committee hearing on Wednesday. Boeing said Pierson’s claim that the production issues were linked to the crashes were “completely unfounded,” noting that no authorities have concluded that production problems contributed to the crashes.