Google Inc has found itself at the centre of investigations in the US into whether it is using its dominance in search advertising to scotch competition.

At least three state attorneys general have started antitrust investigations into Google, a source familiar with the matter said.

The source declined to elaborate on the details of the investigations by the attorneys general of California, Ohio and New York as they were still in the early stages.

The attorneys general investigation into Google was first reported by the Financial Times, citing people familiar with the investigations.

The news of the attorneys general investigation emerged on the same day the Wall Street Journal reported that the internet search giant is about to receive the civil equivalent of a subpoena from the US Federal Trade Commission as part of a probe into the company's internet search business.

The company, which dominates US and global markets for search advertising, has been accused by competitors of favouring its own services over rivals in its search results.

Google and the FTC declined to comment on the Journal report.

The attorneys general of California and New York declined comment while the attorney general of Ohio was not immediately available for comment.

The FTC plans to send the civil investigative demand with a request for more information, the civil equivalent of a subpoena, within five days, according to the Journal report.

US antitrust regulators have been concerned about Google's dominance of the Web search industry, and it has been under investigation by the European Commission since last November.