Labor groups blasted the electric vehicle maker in a letter to Tesla executives, claiming union-supporters may have been dismissed because they raised issues about wages and safety in the plant. While Tesla insists the workers were let go because of poor performance, some workers among the 400 to 700 employees fired, claim they had consistently strong performance reviews before being let go. The National Labor Relations Board is also looking into whether Tesla harassed workers distributing union materials.

Tesla employs about 33,000 workers. The firings ranged from factory workers in Fremont to engineers at its Palo Alto headquarters. The company, which lost $336 million in the second quarter of the year, is preparing to ramp up production for its lower-cost Model 3 sedans. It has about 450,000 reservations for the all-electric vehicles and produced only 260 last quarter.