Lawmakers Probe Web Tracking

"This is an obvious privacy violation even when the eavesdropper does not know your identity," she said. "The issue we have is with the interception itself. We think people simply do not expect a middleman to be sitting between them and the Web sites they visit."

This week, Markey, committee Chairman John D. Dingell (D-Mich.) and Rep. Joe L. Barton (R-Tex.) sent a letter to Embarq seeking to know, among other things, where the firm tested the technology, with how many subscribers and why it chose to proceed without first asking customers whether they wanted to opt in.

A "minuscule" subset of customers was targeted in the test, which ended in March, said a source familiar with the test who spoke on condition of anonymity.

Embarq spokeswoman Debra Peterson said company officials "are reviewing [the letter] for an appropriate response."

Last month, Charter Communications of St. Louis, the country's fourth-largest cable operator, backed off a plan to partner with NebuAd, citing "questions about this service" from customers.

Dykes said that "clearly quite a few" service providers have suspended their arrangements, which proves the need to "better educate the public" about NebuAd's privacy policies. For instance, he said, providers can offer consumers direct online notice that their browsing activity will be tracked for ad purposes and that they may opt out of this.

The new technology, said Emily Riley, online advertising analyst with Jupiter Research, "is almost too exciting for marketers to resist" but also is "a minefield" for privacy because of what she calls the creepiness factor.

"If you remind people that you're tracking them -- 'Hey, I know that you recently started wearing medium T-shirts instead of large. Did you lose weight?' That's creepy," she said.

Staff researcher Madonna Lebling contributed to this report.