WASHINGTON (AP) â€" AT&T Mobility, the nation's second-largest cellular provider, said Friday it's no longer attaching hidden Internet tracking codes to data transmitted from its users' smartphones. The investigative website ProPublica also discovered that Twitter's advertising arm was using Verizon's tracking codes, which could be used to build a dossier about a person's behavior on mobile devices. Some cell providers already collect and store the approximate location of their subscribers' phones, according to government documents from 2010. Consumers' interest in privacy and their digital anonymity has intensified in recent years, following revelations by former National Security Agency analyst Edward Snowden. Top secret NSA documents he leaked to journalists revealed the NSA was collecting the phone records and digital communications of millions of citizens not suspected of a crime, prompting congressional reform.