U.S. regulators looking at whether Google manipulates its search results to favor its own products have expanded the probe to include Google+, the search giant's new social networking tool, a source familiar with the probe said on Friday.

Google+, which was launched in June, offers many of the capabilities available on Twitter and on Facebook.

Google announced on Tuesday new features aimed at making search results more personalized. As part of the changes, photos and posts from Google+ will increasingly appear within the search results.

A Google spokesman, however, said that the Federal Trade Commission had not asked about Google+ or indicated it was looking into the new social network as of this week.

Google has been broadly accused of using its clout in the search market to shut out rivals, like travel search, by putting them low in search results. The source said that the FTC was roughly in the middle of what could be an extended probe.

The Federal Trade Commission declined comment. Bloomberg earlier on Friday reported the expansion of the Google probe, citing two people familiar with the situation.

The Electronic Privacy Information Center had asked the FTC this week to look at Google's decision to include information from Google+ in Google's search results.

Twitter had also protested the change. Twitter's general counsel, Alex Macgillivray, a former Google executive, said in a Tweet on Tuesday that Google's changes "warped" Web searches and represented a "bad day for the Internet."

Google said Friday that it believed including Google+ in search results was good for users. "We believe that our improvements to search will benefit consumers by better surfacing social content, and the great thing about the openness of the Internet is that if users don't like our service they can easily switch to another site," the company said in a statement.