U.S. Senator Jay Rockefeller IV (D-W.Va.) is scrutinizing the data collection practices of About.com, Health.com, Investopedia.com, and nine other websites as part of his investigation into brokers that gather and share consumer information, the U.S. Senate Commerce, Science, and Transportation Committee announced Wednesday.

Rockefeller, the chairman of the panel, sent letters seeking details on possible data collection through questionnaires, surveys, and other means. The letters went to:

These websites “are among the most popular online sources of advice and information on health, family, and personal finance issues—and they may collect detailed or sensitive information about a consumer’s health or financial status,” according to a committee news release. Rockefeller issued the letters after he learned that data brokers may receive consumer information from online questionnaires and surveys, among other sources. The senator said in his letter that data brokers, whom he has been investigating since October 2012, have declined to identify the exact providers of the information they obtain.

“Transparency regarding website information practices is increasingly important as greater numbers of consumers turn to the Internet for advice and information,” Rockefeller said.

About.com, American Media, The Motley Fool, and Condé Nast representatives said their companies have received Rockefeller’s letter and intend to respond. Representatives of the other companies didn’t immediately respond to requests for comment. In statements, About.com, American Media, and Condé Nast representatives added that their companies take consumer privacy seriously.

In addition to Rockefeller, the Federal Trade Commission is conducting its own study of data brokers. The FTC plans to release a report this year.

FTC Chairwoman Edith Ramirez last month put companies that house vast amounts of consumers’ personal data on notice. The agency is ready to take action when consumer privacy is under threat, she said.

“The FTC recognizes that the effective use of big data has the potential to unleash a new wave of productivity and growth,” Ramirez said. “Like the lifeguard at the beach, though, the FTC will remain vigilant to ensure that while innovation pushes forward, consumer privacy is not engulfed by that wave.”