Google is to be investigated by the Federal Trade Commission as to whether it abuses its position in search dominance to steer users to its own sites and services.
According to the Wall Street Journal (WSJ), the Federal Trade Commission (FTC) is expected to open a formal inquiry in the next week.
The civil probe is the most serious legal threat yet to Google, though it wouldn't necessarily lead to any federal allegations of wrongdoing against it, said the WSJ.
The search giant has been accused of using its dominance in search to unfairly promote its other products - such as mapping, shopping and travel websites - above those of competitors.
"Google is abusing its dominant position in search to stifle competition and capture more control over the flow of information and commerce online," said FairSearch.org, an organisation that represents Google critics such as Microsoft and Expedia.
"Google's practices are deserving of full-scale investigations by US antitrust authorities," FairSearch.org said.
The European Commission is already investigating Google's practices over antitrust allegations, to decide if Google uses its dominance to stifle competition.