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The two search companies announced a joint two-week trial earlier this month which saw Google serve targeted advertising on Yahoo's search results page.

Both companies claim to have informed the DOJ prior to the start of the trial, and to have answered subsequent questions from the industry regulator.

"We informed the Justice Department before we launched this test and we have been responsive to their questions about it," claims Google. Yahoo spokesperson Tracy Schmaler also claims to have "proactively" kept the Department of Justice informed of the trial.

However, the DOJ regularly makes preliminary investigations without them progressing to a full investigation, claims anti-trust attorneys speaking to the Financial Times today.

Sources quoted in the Wall Street Journal claim that a permanent deal with Google could net Yahoo up to $1 billion each year. However, some saw the move as an attempt to pile pressure on Microsoft to increase its bid for Yahoo.