Documents related to a Senate inquiry into Carrier IQ and its smartphone software reveal that Sprint is by far the company’s biggest carrier client in the United States. Sprint stated in a letter to Senator Al Franken, which is now public record, that Carrier IQ software is installed on more than 26 million of its handsets. A similar letter from AT&T states that the mobile tracking software is installed on 900,000 AT&T phones, but the carrier said it is only collecting data from approximately 575,000 of them. Both companies reaffirmed earlier statements claiming they only use Carrier IQ software for diagnostic purposes and not to gather private user data or to track subscribers. “Sprint has not used Carrier IQ diagnostics to profile customer behavior, serve targeted advertising, or for any purpose not specifically related to certifying that a device is able to operate on Sprint’s network or otherwise to improve network operations and customer experiences,” Sprint wrote in its letter. AT&T made similar claims. A link to Senator Franken’s full response to the letters, which includes links to letters from Sprint, AT&T, Samsung and HTC regarding their use of Carrier IQ, follows below. Samsung states in its letter that Carrier IQ is installed on approximately 25 million of its smartphones, and HTC says roughly 6.3 million of its handsets shipped with the software pre-installed.