Google handles nearly two-thirds of Internet search queries worldwide. Analysts reckon that most Web sites rely on the search engine for half of their traffic. When Google engineers tweak its supersecret algorithm  as they do hundreds of times a year  they can break the business of a Web site that is pushed down the rankings.

When Google was a pure search engine, it was easy to appear agnostic about search results, with no reason to play favorites with one Web site or another. But as Google has branched out into online services from maps and videos to comparison shopping, it has acquired pecuniary incentives to favor its own over rivals.

Google argues that its behavior is kept in check by competitors like Yahoo or Bing. But Google has become the default search engine for many Internet users. Competitors are a click away, but a case is building for some sort of oversight of the gatekeeper of the Internet.

In the past few months, Google has come under investigation by antitrust regulators in Europe. Rivals have accused Google of placing the Web sites of affiliates like Google Maps or YouTube at the top of Internet searches and relegating competitors to obscurity down the list. In the United States, Google said it expects antitrust regulators to scrutinize its $700 million purchase of the flight information software firm ITA, with which it plans to enter the online travel search market occupied by Expedia, Orbitz, Bing and others.