The inquiry began at the end of last week, after it was decided that the Federal Trade Commission instead of the Justice Department would conduct the review, said the executive, who asked not to be identified because he had not been authorized to speak. The two agencies split the duties of antitrust enforcement.
An FTC spokesman said Monday that the agency did not comment on pending inquiries.
, involving powerful forces in their respective niches of the online advertising business, after it was announced last month. Those concerns and the deal's size made a preliminary investigation all but certain, according to antitrust experts.
Within a few weeks, perhaps within days, the FTC will decide whether to escalate its investigation into the Google deal, antitrust experts say. That step, known as a "second request" for information, would suggest that the proposed acquisition raises more serious antitrust issues.
Google said it was confident that the deal would withstand scrutiny.
Privacy groups said it was significant that the FTC, the agency that monitors online privacy issues, would be conducting the review.
"We think it's very important that the FTC is taking a look at the Google-DoubleClick deal," said Marc Rotenberg, executive director of the Electronic Privacy Information Center, a privacy rights group.
In the days after the planned merger was announced, Rotenberg's center and two other advocacy groups, the Center for Digital Democracy and the United States Public Interest Research Group, filed a request for the FTC to investigate the privacy implications.
In the complaint, the groups noted that Google collects the search histories of its users, while DoubleClick tracks what Web sites people visit. The merger, according to their complaint, would "give one company access to more information about the Internet activities of consumers than any other company in the world."
Google has built a lucrative business in selling small text ads that appear alongside its search results and on other Web sites. DoubleClick is the leader among companies that specialize in placing graphical and video ads online.
Jeff Chester, executive director of the Center for Digital Democracy, said that decisions made now about the structure of the online advertising industry could have lasting effects on data collection and personal privacy on the Internet, especially if control rests with a "few powerful gatekeepers" led by Google.
Still, privacy issues are not typically the concern of antitrust officials. In reviewing a proposed merger, legal experts say, regulators weigh the likely impact on competition and struggle with tricky technical matters like defining the relevant market to measure.
"To the extent that a reduction in competition could make it more difficult to protect privacy, it could be a consideration," said Andrew I. Gavil, a law professor at Howard University. "But it would have to be linked to competition. Strictly speaking, privacy is not an antitrust issue."
Google, the Internet search giant, is facing questions about its privacy practices not only from advocacy groups in the United States, but also from an advisory panel for the European Union. The company has said it welcomes the debate. It defends its privacy safeguards and says its business is based on consumer trust.
As for the DoubleClick acquisition, Google on Monday repeated its optimism that antitrust regulators would approve the deal.
"We are confident that upon further review the FTC will conclude that this acquisition poses no risk to competition and should be approved," said Don Harrison, a senior corporate counsel for Google.
Harrison pointed to the flurry of deals in recent weeks, after Google announced its bid for DoubleClick on April 13. Later in the month, Yahoo announced it would pay $680 million for the 80 percent of Right Media, an online ad exchange, that it did not already own.
In May, WPP, the big ad agency, said it would, whose ad serving business competes with DoubleClick. And then Microsoft, which pushed for an antitrust investigation of the Google-DoubleClick deal, , an Internet ad company. One of Aquantive's units, Atlas, competes with DoubleClick.
Harrison said that "the online advertising industry is a dynamic and evolving space--as evidenced by a number of recently announced acquisitions." And he added that "rich competition in this industry will bring more relevant ads to consumers and more choices for advertisers and Web site publishers."
Among the competitors that had called for an antitrust review were Microsoft, which had lost out in the bidding for DoubleClick, and AT&T, which distributes services over the Internet like digital television.