The Federal Trade Commission (FTC) is expected to open a formal inquiry within the next several days, the Wall Street Journal said.
The FTC is looking at whether Google manipulates its search results to steer users to its own sites and services.
Google has not commented on the matter.
Google's competitors argue that the search giant, which handles roughly two out of every three internet searches in the US, has used its dominant standing in search to improperly promote its other products, like mapping, shopping and travel websites.
FTC officials privately debated this month whether to allow the agency's Bureau of Competitions to issue subpoenas to Google, and the FTC is now close to moving forward with handing out the court orders, The New York Times reported.
Meanwhile, the Financial Times said that attorneys-general in California, New York and Ohio have also launched antitrust investigations into Google.
The European Commission is already conducting probes into whether Google uses its dominance to wrongfully stifle competition.
In a statement on its website, FairSearch.org, an organization that represents several of Google's critics, like Expedia, Travelocity, Kayak and Microsoft, said: "Google's practices are deserving of full-scale investigations by US antitrust authorities."
Though the FTC probe would be the broadest federal inquiry into Google to date, the company has previously been targeted by US regulators.
Google settled charges with a US policy group in April, which claimed the company deceived users and violated its own privacy policy by automatically enrolling all Gmail users in a social network called Buzz without seeking prior permission.
The company has faced repeated other antitrust inquiries in recent years, many of which have involved proposed acquisitions.
US federal regulators are preparing to issue court orders to Google and other companies as part of a probe into practices in Google's search engine business, US media report.