MEXICO CITY (AP) — U.S. trade authorities said Tuesday they have filed a labor complaint with Mexico over allegations that a Volkswagen auto plant in central Mexico unfairly fired union activists.

The U.S. Trade Representative’s office said Tuesday the complaint includes the temporary suspension of tariff benefits for vehicles and parts produced at the VW plant in Puebla, just east of Mexico City.

The complaint was the 23rd filed for alleged labor abuses in Mexico under the U.S.-Mexico-Canada Agreement, known as the USMCA.

Mexico must investigate the claims and either get the company to correct them, or explain why it won’t take action. For decades, wages in Mexico have been held very low because unions were not allowed to organize freely.

The complaint asked Mexico to investigate whether management at Volkswagen de México, S.A. de C.V. fired or took reprisals against workers “based on their service as union representatives, affiliation with prior union administrations, candidacy in union elections, or engagement in other union activities.”