U.S. regulators are in the early stages of an antitrust probe into whether Google Inc, which dominates web display advertising, has broken antitrust law in how it handles some ad sales, a source told Reuters on Thursday.

The source said that it was unlikely that the Federal Trade Commission had sent out civil investigative demands in relation to the probe, which would be the sign of a formal and more serious investigation.

The FTC wrapped up a previous investigation with Google in January, concluding that it had not manipulated its Web search results to hurt rivals but winning promises that it would change certain behaviors.

(Reporting by Diane Bartz, editing by Ros Krasny)