The four workers who filed a lawsuit against the company last week, claimed they were fired from Google for engaging in legally protected labour organizing, reports CNN Business.
The National Labor Relations Board has begun a formal probe into the complaint.
The tech giant has been accused of "union busting" and retaliatory behaviour after it sacked four employees for allegedly violating the company's data security policies.
In a statement, Google said it dismissed four individuals who were engaged in intentional and often repeated violations of its longstanding data security policies.
"No one has been dismissed for raising concerns or debating the company's activities," said the company on Monday.
Google is in the midst of controversy over its strained relationship with employees.
In an earlier blog post on Medium, an employee activist group, "Google Walkout for Real Change", said that the company is illegally retaliating against prospective union organisers.
"Four of our colleagues took a stand and organised for a better workplace. This is explicitly condoned in Google's Code of Conduct, which ends: 'And remember... don't be evil, and if you see something that you think isn't right speak up.' When they did, Google retaliated against them," the employee activist group wrote in the blog post.
The new CEO of Alphabet Sundar Pichai faces extreme challenges as Google stares at several high-profile external probes into its alleged anti-trust market and data practices from the US to the European Union regulators including internal tensions with staff over discrimination at work and HR transparency.
( With inputs from IANS )