The National Labor Relations Board (NLRB) has filed charges against Amazon, alleging that the e-commerce giant has illegally refused to bargain with a union representing drivers who are frustrated by what they claim are low wages and dangerous working conditions.

Back in August, drivers celebrated what they considered a major win when the NLRB found that Amazon was a joint employer of sub-contracted drivers, cheering “We are Amazon workers!” At that time, Amazon seemed to be downplaying the designation, telling Ars that the union was trying to “misrepresent” a merit determination that the NLRB confirmed was only “the first step in the NLRB’s General Counsel litigating the allegations after investigating an unfair labor practice charge.”

But this week, the NLRB took the next step, signing charges soon after Amazon began facing intensifying worker backlash, not just from drivers but also from disgruntled office and fulfillment workers. According to Reuters, the NLRB accused Amazon of “a series of illegal tactics to discourage union activities” organized by drivers in a Palmdale, California, facility.