WASHINGTON The U.S. Justice Department is investigating possible antitrust implications of Google's two-week test with Yahoo to combine some of their Web search and advertising business, a source informed about the matter told Reuters on Wednesday.
Google and Yahoo separately told Reuters they had informed the Justice Department about their test before it was launched.
In the test, which ends this week, Yahoo uses Google's advertising system to show ads to Yahoo users based on their searches.
The Justice Department is concerned the test may violate antitrust law, the source said, adding that authorities "have initiated an investigation" of it.
The source, who spoke on condition of anonymity, said some of the government's concern focused on a telephone call from Google Chief Executive Eric Schmidt to Yahoo Chief Executive Jerry Yang to offer help in thwarting Microsoft's bid worth around $44 billion.
The test was one of a series of efforts by Yahoo to fend off Microsoft's unwelcome bid.
A second source said the Justice Department was concerned about a longer-term deal between Google and Yahoo, and had an initial inquiry underway into the matter.
A Justice Department spokeswoman would only say that the department was "aware of the collaboration."
Neither Google nor Yahoo has said whether the two-week test will be extended.
"We informed the Justice Department before we launched this test and we have been responsive to their questions about it," said Google spokesman Adam Kovacevich.
Yahoo spokeswoman Tracy Schmaler said: "Yahoo proactively kept the Department of Justice informed of its intentions to conduct this limited test with Google and have provided information to DOJ on the nature of the test."
Microsoft declined to comment.
Google shares closed down 1.5 percent to $546.49 in regular trading on Wednesday, while Yahoo, which posted a better-than-expected quarterly profit on Tuesday, finished down 1.6 percent to $28.08, both on Nasdaq.
Microsoft shares gained 4 percent to $31.45.
The initial Google and Yahoo test is small, covering only 3 percent of Web searches performed on Yahoo, the companies have said.
Google is the top search engine with 63 percent of searches, and No. 2 Yahoo has 17 percent, a combined 80 percent of the market, according to ratings company Hitwise.
Philip Bromiley, who teaches law at the University of California at Irvine, said that kind of clout meant that companies could sharply raise prices.
"Any industry, when you start to see that kind of figure, you're going to have antitrust arguments," he said.
Sources familiar with the matter told Reuters on Tuesday that Yahoo was still in separate talks with News Corp and Time Warner Inc about other types of deals -- all designed to avoid a Microsoft takeover.
Microsoft is prepared to walk away from its bid for Yahoo Inc if the two sides can't agree on a price, Chief Executive Steve Ballmer said in Italy on Wednesday.
"We think the best way to move forward quickly is to come together with Yahoo," Ballmer said. "Hopefully that works. But if it doesn't, we go forward."
Ballmer has set a Saturday deadline for Yahoo's board to accept a deal with Microsoft or face a lower bid that Microsoft would take directly to Yahoo's shareholders. Yahoo's board of directors says Microsoft's cash-and-stock offer is too low.
Yahoo President Susan Decker told Wall Street analysts in a conference call on Tuesday that it was too early to say whether Yahoo would reach a deal to turn over some part of the company's Web search advertising business to rival-turned-Microsoft-counterweight Google.
"It's premature to speculate on what options we may ultimately pursue or whether some form of arrangement might result," Decker said.
(Reporting by Diane Bartz and Eric Auchard; Editing by Tim Dobbyn)