The employees said Thursday they were fired for being part of a group that had drafted and circulated a letter to SpaceX executives in June criticizing Musk, the world's richest person, and urging executives to make the firm's culture more inclusive.

SpaceX did not immediately respond to a request for comment.

Reuters reported in June that SpaceX fired at least five employees who were involved in drafting the letter, which called Musk a "distraction and embarrassment" to the company.

The charges filed Wednesday with the National Labor Relations Board allege that SpaceX fired five employees the day after the letter was revealed, and four others in the two months after.

While two of the employees filed charges on their own, attorneys filed charges on behalf of six others who are proceeding anonymously.

U.S. labor law prohibits employers from firing workers who band together to advocate for better working conditions. When the NLRB finds that firings violated the law, it can order that workers be reinstated and given back pay.

Musk, also the head of electric-car maker Tesla Inc , has been in the headlines over his $44 billion acquisition of Twitter and attempts to remake the social media company after he warned of a potential bankruptcy.

Musk also has been on trial this week over a shareholder challenge to his $56 billion Tesla compensation package.

The letter sent to SpaceX executives in June focused on a series of tweets Musk had made since 2020, many of which were sexually suggestive.

The employees claimed Musk's conduct did not align with the company's policies on diversity and workplace misconduct. They called on SpaceX to publicly condemn Musk's comments and to more clearly define the type of conduct prohibited by company policy.