SAN FRANCISCO — The Federal Trade Commission has opened an antitrust investigation into Google Inc.'s proposed $3.1 billion purchase of ad-management technology company DoubleClick Inc.

The review of the deal was widely expected after Mountain View-based Google announced plans last month to acquire DoubleClick, a company that helps its customers place and track online advertising.

New York-based DoubleClick helps its customers place and track online advertising, including search ads, which Google — more than its nearest search competitors Yahoo Inc. and Microsoft Corp. — has turned into an extremely lucrative business.

DoubleClick had been the target of a fierce bidding war between Microsoft and Google. Though Google commands the bulk of the online advertising search market, the addition of DoubleClick's technology and client network would further its efforts to branch out beyond its core ad offerings.

Don Harrison, the senior corporate counsel for Google, said in a statement Monday that the acquisition "poses no risk to competition and should be approved."

"Numerous independent analysts and academics have determined after looking at this acquisition that the online advertising industry is a dynamic and evolving space — as evidenced by a number of recently announced acquisitions — and that rich competition in this industry will bring more relevant ads to consumers and more choices for advertisers and Web site publishers," Harrison said.

Google turned over basic information about the planned purchase to regulators after announcing the deal. Spokesman Adam Kovacevich said the company would soon turn over additional information sought by the FTC as part of its review.

News of the FTC investigation was first reported by The New York Times.

A phone message left with a spokesman for the FTC late Monday night was not immediately returned.

The Washington-based Electronic Privacy Information Center and other privacy groups had previously asked the FTC to investigate the the privacy implications of the deal.

Earlier this month, Google Chairman Eric Schmidt predicted the company would clear all the necessary regulatory hurdles to complete the acquisition by the end of this year.

Google pointed to four deals made after its bid for DoubleClick on April 13 as evidence that there is plenty of competition in the online advertising market.

On April 30, Yahoo announced its intent to acquire online advertising firm Right Media Inc., for $680 million.

Then in a three-day span in May, AOL LLC said it bought a controlling interest in online advertising company Adtech AG for an undisclosed sum; WPP Group PLC, the world's second-largest advertising and marketing conglomerate, agreed to buy 24/7 Real Media Inc. for $649 million; and Microsoft made a $6 billion bid for online advertising firm aQuantive Inc.