Elon Musk’s SpaceX illegally fired employees because they wrote and shared a letter about their workplace concerns, the U.S. labour board alleged.

A regional director of the National Labor Relations Board issued a complaint against SpaceX Wednesday, alleging that the company illegally interrogated, surveilled and retaliated against workers, agency spokesperson Kayla Blado said in an email. The fired workers include authors of a 2022 open letter protesting “inappropriate, disparaging, sexually charged comments on Twitter” by Musk, according to their attorneys.

The company, formally known as Space Exploration Technologies Corp., didn’t immediately respond to a request for comment.

A trial is set to begin March 5, the NLRB said.

Complaints issued by NLRB prosecutors are considered by agency judges, whose rulings can be appealed to the NLRB members in Washington, and then to federal court. The agency has the authority to order companies to reinstate fired workers and provide back pay, but generally can’t hold executives personally liable for alleged wrongdoing or issue any punitive damages.