(Reuters) - The National Labor Relations Board filed an official complaint against a California warehouse serving some of the largest U.S. retailers after finding evidence it violated workers’ rights to organize, according to a filing by the agency.

The complaint against California Cartage Company, LLC, and an affiliated firm means allegations of wrongdoing submitted by a worker group last year will move forward and be heard by an NLRB administrative law judge in June, the filing showed.

According to the complaint, which consolidated two cases, managers at the Long Beach facility discouraged employees from organizing and threatened them with dismissal in violation of labor law.

The filing says that the California Cartage and its affiliated company must file an answer to the agency’s consolidated complaint by the first week of April.

The case is the latest sign that labor activists are making headway in their efforts to shine a light on what they say are consistently poor working conditions at the ports and warehouses vital to the retail industry’s sprawling supply chain.

The California Cartage facility serves Amazon.com Inc, Lowe’s Companies Inc, New Balance, and Sears Holdings Corp, according to Warehouse Workers Resource Center, or WWRC, the Ontario, California-based labor group that submitted claims of wrongdoing to the NLRB last year.

Sears declined to comment. New Balance did not immediately respond to a request for comment. Al Latham, a lawyer representing California Cartage and Orient Tally Company Inc, the affiliated firm named in the complaint, said the companies could not comment on ongoing litigation. Amazon and Lowe’s did not immediately respond to emails requesting comment.

None of the retailers were named in the NLRB complaint.

Celene Perez, co-director of WWRC, said the decision by the NLRB to issue an official complaint was significant because it showed that the company had violated the workers’ rights to join their co-workers in improving their working conditions.

“What is also significant is that it applies to temp workers as well as direct employers. The complaint made very clear that California Cartage and Orient Tally violated the rights of both workers,” said Perez.

Workers can file labor complaints with regional directors of the NLRB, an independent federal agency, and it is up to the director to issue a complaint and post a hearing after an investigation. The NLRB filing on the California Cartage case was signed by Olivia Garcia, the regional director of the agency in Los Angeles.