The Federal Trade Commission (FTC) has begun an inquiry into the planned sale of DoubleClick to Google for US$3.1 billion, according to reports.

An intervention by federal investigators appeared likely due to the size of the deal and the crossover in the companies' interests in the online advertising sales market.

According to reports by Reuters, The New York Times and others, the inquiry began last week after the two main federal industry regulators, the FTC and the US Justice Department, agreed it was best for the FTC to handle the case.

Any investigation is likely to be carried out on antitrust grounds, but first the FTC has to decide if a detailed investigation is required. Any investigation is likely to centre on the fact that, while Google collects the search histories of its billions of users, DoubleClick tracks which Web sites people visit. That combination is potentially a powerful tool.

Back in April, Microsoft had led the way in the bidding for DoubleClick but, once the bidding went above US$2 billion, the company lost interest. It is likely that the prospect of a battle with regulators over any purchase may also have dampened Microsoft's interest.

Google is currently optimistic as, earlier this month, it announced that its first-quarter profit had risen by nearly 70 percent, leading to its results beating Wall Street expectations.

Colin Barker reported for ZDNet UK from London