Earlier this week, the European Commission acknowledged that it had begun looking into three antitrust complaints filed against Google. In response, Google has defended its search policies but has also blamed Microsoft for triggering the investigation, because Microsoft owns one of the companies that complained and is linked to another.

"Google is telling reporters that antitrust concerns about search are not real because some of the complaints come from one of its last remaining search competitors," Dave Heiner, vice president and deputy general counsel at Microsoft, wrote in a blog post on Friday.

There's a reason for that, he said. "Complaints in competition law cases usually come from competitors," he wrote. "Google hasn't been shy about raising antitrust concerns about Microsoft in the last few years, either. ... Ultimately what's important is not who is complaining, but whether or not the challenged practices are anticompetitive."

Microsoft, which has faced its share of antitrust complaints, is "among the first to say that leading firms should not be punished for their success," he said. "Our concerns relate only to Google practices that tend to lock in business partners and content (like Google Books) and exclude competitors, thereby undermining competition more broadly."

From its side, Google on Thursday generally described its search ranking technology and said it never manually chooses results. One of the companies that complained to the commission, Foundem, suggests otherwise. Foundem believes it was essentially blacklisted from Google's search results for a while because the company competes with Google.

Google also stressed the challenges involved in processing hundreds of millions of queries a day. The company didn't reveal anything new about the way it handles search, but it said news of the Commission's investigation had prompted "lots of questions" about how Google's ranking works.

Google dominates the European search market, with about 85 percent market share in many countries, while Microsoft's Bing typically has closer to a 3 percent share.