SpaceX is facing a complaint from the National Labor Relations Board (NLRB) that alleges the company illegally fired employees for writing an open letter that criticized CEO Elon Musk.

Last year, a group of SpaceX employees drafted an open letter to express concerns about how Musk’s behavior “is a frequent source of distraction and embarrassment.” At the time, Musk was in the process of trying to acquire Twitter, which he later renamed X after taking control of the company. SpaceX subsequently fired the employees who helped write the letter.

In the complaint filed on Wednesday, the NLRB accuses SpaceX of interrogating workers about their involvement with the letter and told employees not to discuss these interviews. NLRB spokesperson Kayla Blado told Reuters that SpaceX violated the federal right of workers to collectively push for better conditions.

The labor agency claims SpaceX “created an impression of surveillance” by reading and showing screenshots of messages between employees, adding that the company attempted to prevent employees from handing out the open letter. SpaceX also allegedly “invited employees to quit and threatened discharge” if they participated in organized activities.

The NLRB is asking SpaceX to post a notice about employee rights for 120 days and wants the company to write letters of apology to each of the employees it fired, among other things. A hearing with an NLRB administrative law judge (ALJ) is scheduled for March 5th, 2024, provided that SpaceX doesn’t try to settle the charges. Whatever decision the judge makes can be appealed to the board and taken to the federal appeals court.