Ranking Democrats Henry Waxman and G.K. Butterfield of the U.S. House Energy and Commerce Committee sent letters on Thursday to developers of iOS apps, asking for information on how the programs collect and store user data.The lawmakers are seeking the information as part of an inquiry into privacy and security practices related to mobile phone applications, according to the committee's Democratic website.The inquiry comes amid growing concern about mobile privacy, which remains largely unregulated despite the explosion of smartphone usage. In mid-February, after reports that the social networking app Path was uploading users' address books without their knowledge, Waxman and Butterfield sent a letter to Apple CEO Tim Cook, requesting clarification of the privacy protections the company demands of app developers. Last week, the lawmakers notified Cook that they weren't satisfied with the answers and asked Apple representatives to brief them in person.The letters sent Thursday to the developers of apps such as Path, Facebook, Instagram, Foodspotting and others asked developers to explain how their apps use personal information, including address books, email accounts, calendars, photos and unique device identifier numbers. The committee members said they expect answers by April 12.Cameron Scott covers search, web services and privacy for The IDG News Service. Follow Cameron on Twitter at CScott_IDG.