Updated

Eleven of the nation's largest website operators defended their privacy practices to lawmakers, saying it is impossible for them to monitor all the tracking technologies their sites install on visitors' computers.

The operators, including Microsoft Yahoo and AOL, say they are improving disclosures about online tracking and offering users more ways to protect their privacy. But they say that eliminating tracking is technically difficult and economically impractical, because the targeted advertisements supported by tracking allow the operators to offer free content.

"It is technically impossible for Yahoo! to be aware of all software or files that may be installed on a user's computer when they visit our site," Anne Toth, Yahoo's vice president of global policy and head of privacy, wrote to U.S. Reps. Edward Markey (D-Mass.) and Joe Barton (R-Texas).

The lawmakers are co-chairmen of the House Bi-Partisan Privacy Caucus. In August, they requested information about data-collection practices from the 15 websites identified by The Wall Street Journal's What They Know series as installing the most tracking technology on visitors' computers.

The other responding companies were AT&T, CareerBuilder.com, Comcast, Merriam-Webster.com, News Corp.'s MySpace.com, About.com, Photobucket.com and Verizon Wireless.

In an e-mail, Rep. Markey said the responses raise concerns, particularly about "complicated and laborious" privacy policies Internet users must navigate to learn how their data is being used and shared. Mr. Markey said he wasn't satisfied that "consumers are able to effectively shield their personal Internet habits and private information from the prying eyes of online data gatherers."

Read more of this story at the Wall Street Journal.