SpaceX illegally fired eight workers who wrote an open letter laying out their concerns with the management and culture at Elon Musks rocket company, according to prosecutors at the National Labor Relations Board.

The NLRB officials allege in a complaint filed Wednesday that SpaceX interrogated workers about the letter and pressured them not to distribute it, according to board spokesperson Kayla Blado. The letter had directly criticized Musks behavior on social media.

The complaint also claims the company invited employees to quit and threatened them with termination for exercising their rights, Blado said.

SpaceX, which is based in Hawthorne, California, did not immediately respond to a request for comment.

It is against the law for employers to retaliate against workers for exercising protected concerted activity, or banding together with other workers in hopes of improving their working conditions. Employers who violate the law can be ordered to offer reinstatement or other measures to make workers whole.

Late last year an attorney filed charges with the labor board against SpaceX alleging the company had fired several workers for speaking up against SpaceXs failure and refusal to address the culture of sexism, harassment and discrimination that permeates the workplace. Employees had written the open letter in 2022 and cited some of Musks tweets on his social media platform now known as X, according to the charges.

The decision by the NLRBs general counsel to pursue a case against SpaceX means investigators at the board looked into the allegations and found merit in them. Barring a settlement between the board and the company, the case would go to a hearing in which witnesses are called for both sides.