Officials at the Justice Department declined a Washington Post request for comment but legal analysts said they believe the inquiry has found cause for concern, The Post reported.

The Justice Department has asked for "civil investigative demands," which is usually not done "without having identified significant issues," M.J. Moltenbrey, a former director of civil non-merger enforcement at the Justice Department, told the newspaper.

Since the deal was struck in early June, Google and Yahoo! have said they would wait for a Department of Justice review before acting on it. The deal, which Yahoo! said is worth $800 million annually, allows the companies to share ad revenues.

In the deal, Google would supply ads for some Yahoo! Internet searches, the Post reported.