The National Labor Relations Board has opened an investigation into worker conditions at Tesla’s Fremont, Calif., manufacturing plant.

The NLRB filed a complaint Thursday based on charges documented in April by workers and the United Auto Workers union claiming unfair labor practices.

Tensions have been increasing as Tesla scales up the factory’s production schedule to meet ambitious goals for the Model 3.

The UAW and Tesla workers made several complaints including employees at the Fremont plant being interrogated over their union activities. They’ve claimed Tesla management has made efforts to prevent such discussions, and has ignored worker safety concerns.

Tesla workers since then have been speaking out about factory working and safety conditions. They’ve also taken their employer to task for having them sign a confidentiality agreement that could result in termination if they speak publicly about these issues.

Tesla issued a statement dismissing the NLRB complaint and vowing to fight it:

“As we approach Labor Day weekend, there’s a certain irony in just how far the UAW has strayed from the original mission of the American labor movement, which once advocated so nobly for the rights of workers and is the reason we recognize this important holiday.

Faced with declining membership, an overwhelming loss at a Nissan plant earlier this month, corruption charges that were recently leveled against union leaders who misused UAW funds, and failure to gain traction with our employees, it’s no surprise the union is feeling pressured to continue its publicity campaign against Tesla.

For seven years, the UAW has used every tool in its playbook: misleading and outright false communications, unsolicited and unwelcomed visits to the homes of our employees, attempts to discredit Tesla publicly in the media, and now another tactic that has been used in every union campaign since the beginning of time–baseless ULP filings that are meant only to generate headlines. These allegations, which have been filed by the same contingent of union organizers who have been so outspoken with media, are entirely without merit. We will obviously be responding as part of the NLRB process.”

Tesla sees these moves as much more about UAW’s strategy to win a union contract rather than legitimate worker concerns over factory conditions.

There may be something behind worker safety concerns, according to a study released by Worksafe in May. The study looked at information from 2014 and 2015, with preliminary data from 2016, that found injuries at the Fremont plant to be much higher than the industry average. It was based on filings made with the federal Occupational Safety and Health Administration.

SEE ALSO:  Tesla, Its Workers, and Alleged Union Organizers Tussle Over Conditions At Fremont Factory

The UAW so far has failed to find enough support to gain a union election victory with Fremont workers.

The electric carmaker is required to file a response to the NLRB on or before Sept. 14, 2017.

There’s also a hearing scheduled for Nov. 14 of this year at the NLRB’s Oakland, Calif., regional office. An administrative law judge will hear testimony on the allegations made in the complaint.