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, their attorneys wrote when they brought the case in 2022.

The NLRB’s complaint alleges that SpaceX management told workers it had terminated the employees because of their open letter, restricted others from distributing it, and threatened terminations if they engaged in collective action, Blado said.

“At SpaceX the rockets may be reusable but the people who build them are treated as expendable,” one of the workers, Paige Holland-Thielen, said in an emailed statement. “I am hopeful these charges will hold SpaceX and its leadership accountable for their long history of mistreating workers and stifling discourse.”

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. 

In June 2022, a group of employees circulated an open letter though internal SpaceX communication channels criticizing Musk’s online behavior and calling on the company to denounce and distance itself from his public comments.

“Elon’s behavior in the public sphere is a frequent source of distraction and embarrassment for us, particularly in recent weeks,” the letter stated.

Shortly after the letter circulated within SpaceX, multiple employees associated with drafting the letter were fired.

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.

Federal law protects employees’ right to communicate and protest collectively about their working conditions, with or without a union. 

While Musk has declared himself a “free speech absolutist,” his companies have repeatedly been accused by the US government of trying to silence workers. Last year, SpaceX settled a claim that it illegally tried to stifle an employee’s speech. Separately, Musk’s social media company X settled with a former employee who a NLRB regional director had concluded was illegally fired for protesting a return-to-office mandate. NLRB members have also ruled that Musk’s electric carmaker Tesla Inc. illegally fired an activist and that Musk threatened workers on social media; Tesla is appealing in federal court.