Google employees at the tech giant's headquarters in Mountain View, California, walk off the job to protest the company's handling of sexual misconduct claims.

The U.S. National Labor Relations Board has started a new investigation into Google's labor practices.

An agency spokesperson confirmed to CNBC Monday that the probe, which will include whether Google violated labor laws when it recently fired four employees, has officially commenced. It will also look at whether Google discouraged employees from engaging in union activity. The investigation is expected to take roughly three months and be conducted by its regional staff based in Oakland.

A Google spokesperson provided the same statement it's been using since the firings of the four employees:

"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," the statement says. "No one has been dismissed for raising concerns or debating the company's activities."

The latest investigation comes after four Google employees filed a federal complaint with the NLRB on Dec. 5, alleging unfair labor practices, which would violate a settlement made by Google. It also comes as Google's parent company, Alphabet, just got a new leader in Sundar Pichai. Google now faces another federal investigation into its labor practice just months after a separate settlement with the NLRB.

In that settlement, a Google employee filed a federal complaint last year, alleging the company restricted free speech and fired him for expressing conservative views. The settlement, which Google reached with the NLRB in September, required the company to post the list of its policies at its headquarters in Mountain View, California, as well as its Nest Labs offices in Palo Alto, California. The policies included the point that employees had a right to organize and not be retaliated against. However, in settling, Google said it did not admit it made any violations.

The latest investigation stems from employee uproar over the interrogation and subsequent firing of employees Rebecca Rivers and Laurence Berland, who had been placed on sudden and indefinite administrative leave in November for allegedly sharing sensitive information.

After that, Berland and Rivers held a rally in San Francisco that drew in roughly 200 Google workers, demanding the company reinstate the two employees and stating they were placed on leave in retaliation for their activism against the company's handling of hate policies and immigration issues.

The week of Thanksgiving, Google fired four employees, including Berland and Rivers, claiming they shared confidential documents and breached security. In an internal memo, the company's security and investigations team called it a "rare" case.

The four got public support from presidential front-runners Sens. Bernie Sanders and Elizabeth Warren, who took to Twitter to bash Google for alleged "anti-union" actions.

The group's complaint was addressed to Alphabet and Pichai, Google and the company's founders Larry Page and Sergey Brin. Brin and Page stepped down from their official company roles as President and CEO, respectively, last week.