Google is under investigation by the US National Labor Relations Board, CNBC reported on Monday.

The investigation follows the recent firings of four former Google employees named Laurence Berland, Paul Duke, Rebecca Rivers, and Sophie Waldman. All four former employees were involved in internal activism during their time at Google, and they encouraged their colleagues to join in protest over issues like the company’s plan to build a censored version of its search engine in China or its bids on Defense Department contracts. If Google is found to have fired these employees in relation to their activism, it could be found in violation of federal labor law.

The four employees filed a charge with the NLRB on December 5th alleging unfair labor practices. Google has been under investigation by the NLRB before, including a recent probe that was settled only two months ago. In this September settlement with the agency, Google promised that it would allow its employees to unionize and speak to the press, among other pledges. The December charge alleges that Google violated the terms of that settlement.

According to CNBC, the NLRB’s investigation will focus on whether Google violated any labor laws by firing those activist employees and if it discouraged its employees from unionizing. When employees file a charge with the NLRB, the agency must open an investigation to determine if it should take formal action and file its own complaint. The agency’s Oakland staff will be spearheading the investigation that’s expected to take about 90 days to complete.

When asked for comment by The Verge, Google replied with the same statement it’s used since the firings: