Washington -- The Justice Department has quietly hired one of the nation's best-known litigators, former Walt Disney Co. vice chairman Sanford Litvack, for a possible antitrust challenge to Google Inc.'s growing power in advertising.

Mr. Litvack's hiring is the strongest signal yet that the U.S. is preparing to take court action against Google and its search-advertising deal with Yahoo Inc. The two companies combined would account for more than 80% of U.S. online-search ads.

Google...