Google Inc. is the target of an antitrust investigation launched today by the European Commission over allegations that the company lowered the rankings of natural search results of competing search companies.

The commission, which is the executive arm of the European Union, did not specify the source of the antitrust complaints. But in February, Google said it had been notified by the commission of complaints from three European search services: Foundem, a price comparison service in the United Kingdom; Ciao from Bing, another European price comparison service, this one backed by Microsoft Corp.; and EJustice.Fr., a French legal search engine.

The commission says it will look into complaints that Google gave preference in search results to its own search services in order to shut out competitors. The commission also plans to investigate whether Google lowered the quality score for sponsored links to the competing services. A quality score is how Google measures the relevance of an ad to a search query. Google uses quality scores as part of Google AdWords, the bidding system for ads on the search engine.

Google will work closely with the commission to answer its questions, the company said in a blog post today. The company also defended its search results. “It may seem obvious, but not every web site can come out on top, or even appear on the first page of our results, so there will almost always been web site owners who are unhappy about their rankings,” say the authors of the post, Susan Wojcicki, senior vice president of product management, and Udi Manber, vice president, engineering.