Despite the opposition to the deal, some legal experts suggested that the appointment of Mr. Litvack, who has a reputation for independence, might simply be an effort by the Justice Department to deflect any political fallout from its ultimate decision. He served as the Justice Department’s top antitrust lawyer in the administration of President Jimmy Carter.

The Justice Department has been criticized by some in Congress for its approach to antitrust enforcement. “The Justice Department is looking for political cover one way or the other,” said Jeff Chester, executive director of the Center for Digital Democracy, a public interest group that has opposed the deal. “That’s clearly what’s Sandy’s role is here.”

Mr. Litvack, the former vice chairman of the Walt Disney Company, recently left Hogan & Hartson, where he was a partner, to join the Justice Department, a spokeswoman for the law firm said on Tuesday. The Justice Department refused to confirm Mr. Litvack’s appointment. Two people with knowledge of the investigation, who spoke on condition of anonymity because they were not authorized to discuss it, confirmed the appointment, which was first reported late Monday by The Wall Street Journal.

Google and Yahoo have said their deal does not violate antitrust law. The companies voluntarily agreed to postpone putting it into practice for three and half months to give regulators time to scrutinize it. Eric E. Schmidt, Google’s chief executive, recently said that the companies were planning to go forward with the deal next month, even if it hadn’t been cleared by regulators by then.

Google and Yahoo both declined to comment on the antitrust review. Both companies said it would be premature for regulators to block the deal before it was completed.

If the deal is blocked, the fallout for Google will be strategic, said Benjamin Schachter, an analyst with UBS Securities.

Yahoo has said that the agreement will bring it an additional $250 million to $450 million in operating cash flow in the first year.

The fallout of a deal being blocked would be financial and immediate, Mr. Schachter said. Yahoo shares, which are at a five-year low, could drop even further, leaving the company vulnerable to a new bid from Microsoft, or from another party, Mr. Schachter added.