Even as Google (Nasdaq: GOOG) is planning to complete its $12.5 billion acquisition of cell phone maker Motorola Mobility Holdings before Wednesday, more trouble may loom on the horizon for the search engine and Internet commerce giant.

U.S. antitrust regulators told EBay Inc. and Yelp Inc. to provide information on whether Google is competing unfairly, speeding up a probe that began a year ago, people familiar with the matter told Bloomberg news.

The shopping and review websites were among about a dozen companies that received civil investigative demands, which are similar to subpoenas, from the Federal Trade Commission last week, said a person familiar with the matter who wasn’t authorized to speak publicly.

In its requests, the FTC is seeking examples of Google failing to follow through on promises to direct consumers to websites that compete with its own services, according to the person and three others familiar with the matter, who also weren’t authorized to publicly comment.

Agency officials also asked if Google, the world’s most popular search engine, is selling rivals prime advertising space on search results pages, according to the four.

Companies receiving demands also include NexTag Inc. and TheFind Inc., which run shopping websites, two of the people said.

Mistique Cano, a spokeswoman for Mountain View, California- based Google, and Cecelia Prewett, an FTC spokeswoman, declined to comment.

The FTC, which has sent out previous legal demands to Google and companies including Microsoft Corp., is examining rivals’ complaints that Google favors its own services in search results. In November, Gary Reback, a lawyer who represents Internet companies, including NexTag and TheFind, urged the agency to immediately seek information from shopping websites.

‘Great Step’

The new demands for information are “a great step in the right direction,” Reback said. “I just don’t know how big of a step it is.”

Reback declined to say whether his clients received the FTC demands. Dave Cook, vice president of marketing for TheFind, declined to comment on the FTC requests. Luther Lowe, a spokesman for Yelp, declined to comment, as did Shannon Eis, a NexTag spokeswoman. Johnna Hoff, an EBay spokeswoman didn’t immediately respond to an e-mail and phone calls.

Last month the agency hired Beth Wilkinson, a top Washington litigator, to run the antitrust investigation, signaling that the agency may be preparing to sue Google.

Joaquin Almunia, the European Commission’s antitrust chief, told reporters yesterday that Google has a “matter of weeks” to resolve a probe and avoid possible fines over allegations of discriminating against competitors.

Motorola Deal

Motorola outlined the timetable in documents it filed Monday following Saturday’s approval of the deal by regulators in China. That approval removed the final hurdle preventing Google from closing the biggest deal in its 14-year history.

The two companies announced the deal nine months ago.

Google prizes Motorola Mobility Holdings Inc. for its portfolio of more than 17,000 patents.

Owning the rights to those patents will provide Google and the mobile device makers who rely on its Android software with valuable weapons in an intellectual arms race with Apple Inc., Microsoft Corp. and other technology giants.

But investors are worried Motorola’s business making phones will trim Google’s profit margins.

(Bloomberg news and The AP contributed to this report.)