Expedia and TripAdvisor have just filed complaints about Google’s travel search features as part of a larger antitrust probe the search giant is facing in the European Union.

Altogether the European Commission has received 13 formal complaints, including the new complaints brought by the online travel companies, EC spokesman Antoine Colombani told press today. The commission’s probe of Google began in November 2010.

Sources for Agence France-Presse said the Expedia and TripAdvisor complaints centered around Google Flight, a new service that launched in September 2011 and didn’t include destinations outside the U.S. until just two weeks ago, when Google Flight opened trips to Europe and other regions.

At the end of 2011, Google chairman Eric Schmidt paid a visit to Brussels to meet with European Commissioner Joaquín Almunia in person. At that time, a Google spokesperson told VentureBeat the company had already turned over thousands of documents as part of the investigation and did not expect any formal censure from the EC.

The probe kicked off when several parties brought complaints against Google for taking unfair advantage of its domination in the world of web search. These parties claimed Google was decreasing the search ranking of unpaid search results. For example, the claims stated that Google might lower the ranking of a shopping and product search website while increasing the ranking of Google Shopping results.

Another allegation is that Google set a lower Quality Score for its competitors’ sponsored links (Quality Scores help the company to set its ad prices; a lower score would mean a lower ad price). Finally, the search company is accused of setting up “exclusivity obligations on advertising partners, preventing them from placing certain types of competing ads on their web sites, as well as on computer and software vendors, with the aim of shutting out competing search tools.”

The EC’s office should be ready to make public its findings as soon as this month. Saying a preliminary report may be completed after Easter, Alumnia told press last week, “We want to advance in our investigation, but we want to advance on a solid basis, not because of a letter or some pressures.”