The Federal Trade Commission has yet to inform Google it is under investigation and no formal steps -- such as subpoenas issued or demands for documents -- have been taken, the Times reported Saturday.
Sources told the newspaper the FTC was looking into Google's advertisement bundling, specifically to see whether it included agreements not to do business with rivals.
One source described as a person in the advertising business said FTC staff members had contacted him with questions focusing on Google's Ad Exchange service, which allows companies to bid on advertising slots that target specific audiences, the Times said.
The FTC recently dropped an antitrust case it was building against Google, with the agency's commissioners voting unanimously not to proceed with formal charges against the company. The FTC has said it would keep track of Google's business practices to ensure the company does not use its size to create an unfair advantage over rival companies.