Citing an industry executive who had been briefed on the FTC's preliminary antitrust investigation, the newspaper said the inquiry was opened last week after it was determined that the commission, rather than the Justice Department, should conduct the review.

An FTC spokesman Monday told the newspaper the commission does not comment on pending inquiries.

Google announced in April it would purchase the Internet ad-services company from private-equity firm Hellman & Friedman for $3.1 billion -- almost twice the $1.65 billion in stock Google paid for YouTube last year.

Google said the cash deal for DoubleClick was expected to close by the end of the year.

DoubleClick's technology displays ads on Web sites, helps Web sites and advertisers maximize their revenue and monitors ad campaigns. The company has been linked to "spyware" because its Internet browser cookies record which ads users selected and view while on Web sites.

DoubleClick -- whose customers include Time Warner Inc. (NYSE:TWX)'s AOL and News Corp.'s social-networking site MySpace -- says its ad-serving tags are different from spyware.