The four employees claim they were fired for protected labor organizing. Last week, they filed unfair labor practice charges.

Google says it fired the four over repeat violations of its data security policies. In a statement provided to Engadget it said:

"We dismissed four individuals who were engaged in intentional and often repeated violations of our longstanding data security policies, including systematically accessing and disseminating other employees' materials and work. No one has been dismissed for raising concerns or debating the company's activities."

This isn't the NLRB's first Google investigation. A few months ago, as part of a settlement with the agency, Google agreed to remind employees they can freely discuss workplace issues and that they do not have to "avoid controversies that are disruptive to the workplace." In other instances, the NLRB has backed Google's decisions, like the one to fire the diversity memo writer James Damore.

The current investigation is expected to take about three months. Regardless of the outcome, this case is the latest in a string of accusations that Google has a retaliation culture.