A congressional committee wants the nation's largest telecommunications and Internet companies to explain whether they target online advertising based on consumers' search queries and Web surfing habits.

In an expanding inquiry into the state of consumer privacy on the Internet, House Energy and Commerce Committee leaders sent letters Friday to more than 30 companies, demanding to know whether they track where their users go online and use that information to deliver personalized advertising.

Among the companies receiving the letters were Google Inc., Yahoo Inc., Microsoft Corp., AT&T Inc., Comcast Corp., Qwest Communications International Inc., Verizon Communications Inc., Time Warner Inc.'s AOL unit and Time Warner Cable Inc. The companies were given a week to respond.

The letters build on an earlier probe by the committee into "deep packet inspection" technologies, which can track where people go and what they do on the Internet.

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The committee already has been investigating Embarq Corp., an Internet service provider based in Overland Park, Kan., that has been testing technology to track the online behavior of its customers and then serve up targeted ads based on their presumed interests.

Both the House and Senate Commerce Committees have been looking into a small Silicon Valley startup called NebuAd Inc., which developed personalized-ad technology that Embarq and other Internet service providers have used.

But the latest letters broaden the inquiry to encompass much bigger and better-known players on the Internet.

The letters seek details on how many consumers have been tracked, whether those people have been notified and whether they were given the option to "opt out" of it.

The committee also wants to know how the collected information is used and how it is gathered — for example, whether the companies aggregate data from different online applications.

Ari Schwartz, vice president of the Center for Democracy & Technology, a civil liberties group, said he is optimistic that the congressional inquiries could lead to general privacy legislation that would safeguard consumer data gathered both online and offline.

"It's important for Internet users and American consumers in general to understand what information is being collected about them and how it's being used," he said.

The letters were signed by Michigan Democrat John Dingell, chairman of the committee; Joe Barton of Texas, its top Republican; Massachusetts Democrat Edward Markey, who chairs the panel's Subcommittee on Telecommunications and the Internet; and Cliff Stearns of Florida, the subcomittee's top Republican.

"Privacy is a cornerstone of freedom," Markey said in a statement. "Online users have a right to explicitly know when their broadband provider is tracking their activity and collecting potentially sensitive and personal information."

Several companies contacted by the committee said they intend to respond to the letter. In a statement, Yahoo added that its approach to advertising allows it to provide free, personalized content that consumers want as well as ads relevant to them.