Recipients of the letter, including Twitter, Facebook, Foursquare and Path, were asked to provide information about the user data that is collected when consumers download their apps -- and how that data is used.

The letter came after several popular apps, including Path, the social networking tool, were found accessing and uploading address book data from users' iPhones without permission, sparking a massive online controversy last month.

Representatives Henry Waxman and G.K. Butterfield, two Democrats on the House Energy and Commerce Committee, sent the letter to 33 developers that had apps listed under the "iPhone Essential" area, a digital storefront curated by Apple in its App Store. The app makers have until April 12 to respond.

Apple, which was included as a recipient to the most recent letter, has also been tarnished by the scrutiny cast on its app makers; the iPhone and iPad maker has long said it subjects its app developers to a strict review before allowing new products into the App Store.

After the address book controversy emerged last month, Waxman and Butterfield sent a similar letter to Apple to request information about its App Store's consumer protection policies.

The apps inquiry is part of a broader tide of interest gathering on Capitol Hill around Internet privacy issues. In January, legislators including Senator Ed Markey of Massachusetts criticized Google's plans to consolidate its privacy policies, saying it potentially violated a 2010 agreement that the web giant had struck with the Federal Trade Commission to improve its privacy policies.

(Reporting By Gerry Shih in San Francisco; Editing by Tim Dobbyn)

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