The US National Labor Relations Board on Wednesday charged Alphabet Inc.’s Google with violating the National Labor Relations Act by unlawfully surveilling, interrogating and firing activist employees.

The board alleges that in 2019, Google virtually monitored employee activities and questioned them over accessing internal documents and employee calendars. The employees explained that they needed to access them for the purpose of mutual aid and protection.

Two employees were fired for accessing others’ calendars and calendar access rules were put in place, prohibiting employees from accessing others’ calendars without a business purpose. The board noted that the company’s rules discouraged employees from “forming, joining, assisting a union or engaging in other protected, concerted activities.”

One employee, Kathryn Spiers, was terminated for creating a pop-up which said “Googlers have the right to participate in protected concerted activities.” The pop-up appeared whenever employees visited Google’s community guidelines and other webpages. She said on Twitter, “I was illegally terminated for trying to help my colleagues. Workers must unite to tackle the power of the tech giants, it’s the only way to hold them accountable.”

The board has given Google time till December 16 to file its formal response, and the matter is scheduled to be tried before an administrative judge on April 12.