Google expects the state authorities will ask the company about past similar investigations in the US and internationally, senior vice president of global affairs Kent Walker wrote in a blog post Friday.
"I'm proud to stand with my colleague Attorneys General today to announce that we are holding Google accountable for that responsibility and launching an investigation into whether their business practices have undermined free market competition and hurt consumers".
The investigations will initially focus on whether Google is overly dominant in the online advertising market and in internet searches. However, the initial focus of the investigation will focus on online advertising, where the company dominates with 75 percent of all spending on US search ads.
Fifty state attorneys general, led by Texas, have announced an investigation into Google's "potential monopolistic behavior".
Google has been accused of creating an "existential threat" to internet retailers with its grip on digital advertising as USA prosecutors announced a competition probe into the company. Google disclosed on Friday that the department had issued civil investigative demands, which are akin to subpoenas, for all documents in prior antitrust probes.
The announcement comes just days after Attorneys general from seven states and the District of Columbia launched a joint antitrust investigation against Facebook.
Sen. Diane Feinstein did not hold back when questioning representatives of Google, Facebook and Twitter on November 1.
Google responded to the investigation in a blog post written by Kent Walker, senior vice president of global affairs. The two states not participating in the bipartisan probe are Alabama and California.
Reyes emphasized that the state-led effort is not "anti-tech", and argued it is "actually for the benefit of the tech ecosystem to help level the playing field".
Leslie Rutledge, a Republican attorney general from Arkansas, described Google as an "online search engine juggernaut", raising her concern that searches for businesses, including doctors, are colored by the way the tech giant's algorithms and advertising systems work.
The Federal Trade Commission has been conducting its own competition probe of Big Tech, as has the House Judiciary subcommittee on antitrust. "We have always worked constructively with regulators and we will continue to do so", he said. On Friday, Google confirmed that the Justice Department asked the company for information as part of its ongoing tech competition review.
California Attorney General Xavier Becerra's office declined to comment on why it didn't join the coalition of states. The $5 billion penalty against Facebook is the largest ever imposed on any company for violating consumers' privacy and nearly 20 times greater than the largest privacy or data security penalty ever imposed worldwide.