Washington - The Justice Department is investigating whether an advertising tie-up between Yahoo (NASD: YHOO) and Google (NASD: GOOG) violates antitrust laws, Reuters reported. Under a two-week test, scheduled to end this week, Yahoo is using Google’s online ad system to provide relevant ads alongside its search results.

Citing unnamed sources, however, Reuters reported that the Justice Department is looking into the potential implications of a longer deal between the two companies, and specifically how that might affect Microsoft’s (NASD: MSFT) unsolicited takeover bid for Yahoo, which is now valued at around $44 billion.

One source told the news organization that Google CEO Eric Schmidt called Yahoo CEO Jerry Yang, offering to help thwart the bid.

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