Google's termination of the employees led to allegations of retaliation against organizing workers at the tech giant.
An NLRB spokesperson confirmed to ABC News Tuesday that an investigation had been launched following a complaint from the Communication Workers of America, AFL-CIO, labor union.
The charge against employer document, obtained by ABC News, alleges that the fired employees "visibly led and participated" in organized labor efforts "to preserve and improve their working conditions."
Google responded by creating new data classification and other policies, the charge claims, and then investigated the "employee leaders based upon retroactive application of such guidelines" and fired them on Nov. 25 "within minutes of each other."
"Google engaged in all of this unlawful conduct in order to discourage and chill employees from engaging in protected concerted and union activities in violation of the National Labor Relations Act," the document states, adding, "Its actions are the antithesis of the freedoms and transparency it publicly touts."
An NLRB investigation typically takes around three months, an agency spokesperson said.
Google denied the allegations, saying the employees had been fired for violations of "longstanding data security policies."
"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," a Google spokeswoman told ABC News Tuesday. "No one has been dismissed for raising concerns or debating the company’s activities."
The firing of the four employees was announced late last month and came days after Google staff and supporters demonstrated outside of the San Francisco office on Nov. 22 to protest two workers who were placed on sudden leave.
Google Walkout for Real Change, a group representing employees organizing at the company, accused Google of firing the employees "in an attempt to crush worker organizing" in a Nov. 25 statement after the firings were announced.
The turmoil comes amidst a leadership shakeup at Google. Last week, the company announced that effective immediately its founders Larry Page and Sergey Brin would step down as CEO and president, respectively, of Google's parent company Alphabet Inc. and that Sundar Pichai will be the new CEO of both Google and Alphabet Inc.