Twenty-two airline carriers have grounded the newest version of Boeing’s most popular jet, the 737 MAX 8, after two of the planes crashed in recent months. A total of 346 people died in the two crashes. Major U.S. carriers American Airlines and Southwest Airlines, which are heavy users of the model, continued to fly the MAX 8 on Monday. Investigators have not determined the cause of the most recent Ethiopian Airlines crash, but the flight data recorder and the cockpit voice recorder have both been recovered. Some circumstances of the crash were similar to another MAX 8 crash in October. The Federal Aviation Administration issued a “continued airworthiness notification” for the plane on Monday, stating that an investigation was launched into the circumstances of the event and the agency has “not been provided data to draw any conclusions or take any actions.”

U.S. Transportation Secretary Elaine Chao said Monday that U.S. regulators are “very carefully” reviewing the fatal crashes. Chao said she met with the Federal Aviation Administration’s acting chief to review both crashes. “I want people to be assured that we take these incidents, these accidents very seriously,” Chao said.