NEW YORK (AP) — A US labour agency has accused SpaceX of unlawfully firing employees who penned an open letter critical of CEO Elon Musk and creating an impression that worker activities were under surveillance by the rocket ship company.

A Los Angeles-based regional director for the National Labour Relations Board on Wednesday filed a complaint that consolidates eight unfair labour practice charges against SpaceX. The cases stem from the company’s alleged actions following the circulation of the employees’ letter back in June 2022.

The letter, among other workplace concerns, called on executives to condemn Musk’s public behaviour on X — the platform then-known as Twitter — and hold everyone accountable for unacceptable conduct. Musk’s actions included making light of sexual harassment allegations against him, which the billionaire denied.

“As our CEO and most prominent spokesperson, Elon is seen as the face of SpaceX — every tweet that Elon sends is a de facto public statement by the company,” the open letter said at the time. The letter also referred to Musk’s actions as a “frequent source of distraction and embarrassment.”

A total of nine employees were soon terminated for their involvement in the letter, according to a November 2022 filing made on behalf of one of the employees to the NLRB, although only eight are included in Wednesday’s complaint.

In addition to the firings, the complaint accuses SpaceX of interrogating other workers about the letter, announcing that employees were terminated for their participation in the letter and “inviting employees to quit if they disagreed with the behaviour of Chief Executive Officer Elon Musk.”

The complaint also alleges that some were shown screen shots of communications between employees about the letter, which “created an impression among (SpaceX’s) employees that their protected concerted activities were under surveillance.”

Reacting to the board’s complaint, SpaceX sued the NLRB Thursday in federal court. The lawsuit says the NRLB is “unconstitutionally structured” and its action against the company “unlawful.”

Addressing the board’s complaint in its lawsuit, SpaceX lawyers say the employees’ letter caused “significant distraction to SpaceX employees across the country.” It said the employees involved in the letter were fired for violating a number of company policies.

Space X did not immediately respond to The Associated Press’ requests for comment Thursday.

The Hawthorne, California, company has until mid-January to respond to the complaint, according to Wednesday’s filing. The complaint marks the NLRB’s first step towards litigating these allegations and seeking a settlement. If a settlement isn’t reached, a hearing is scheduled to begin on March 5 in Los Angeles.