U.S. labor officials are asking a federal court to force Starbucks Corp. to reinstate a group of activists, escalating the legal battle over the company’s response to the union campaign sweeping through its stores.

In a filing Friday, the National Labor Relations Board’s Phoenix regional director sought an injunction requiring the coffee chain to bring back three employees who the agency alleged had illegally been fired, forced out, or placed on leave.

Workers United, an affiliate of the Service Employees International Union that’s petitioning to represent staff at hundreds of Starbucks cafes, has filed dozens of allegations against the company with the labor board, most of which are still pending. The agency’s prosecutors have found merit in some of those claims, and issued complaints accusing the Seattle-based retailer of illegally firing activists in Arizona and Tennessee.

Starbucks did not immediately respond to an inquiry. The company has previously denied wrongdoing and said that “any claims of anti-union activity” are “categorically false.”

Starbucks retaliated against the three employees because of their involvement with the union and their participation in the NLRB’s own investigations, the agency’s filing alleged. The company’s actions “have irreparably harmed, and are continuing to harm, employees,” the agency said, including by creating “an atmosphere wherein employees fear retaliation and discharge” on a “daily basis” if they show support for the union.

The filing asked that Starbucks, along with being forced to offer the three employees reinstatement, also be required to participate in a video recording of a high-level company official reading the court’s order out loud, or listening to the order being read, and to share that video with its employees across the country.

U.S. labor law prohibits companies from retaliating against workers for taking collective action to improve their working conditions, including union organizing. But the labor board, which prosecutes alleged violations of that law, has no authority to make companies pay punitive damages, and disputes over alleged retaliatory firings can drag on for years, hampering organizing efforts even if the employee eventually prevails.

NLRB regional offices investigate claims and, if they find merit in them and can’t reach a settlement, issue complaints which are then considered by agency judges. The judges’ rulings can be appealed to NLRB members in Washington and from there to federal court. The NLRB’s top prosecutor, General Counsel Jennifer Abruzzo, has said she plans to “aggressively” seek federal court injunctions to get wrongly fired employees back to work more quickly.

Workers United has prevailed in votes at a couple dozen Starbucks stores, including Colorado and Virginia sites where ballot victories were announced Friday.