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What does your mobile carrier know about you?

A 2010 Justice Department memo obtained by the ACLU shows what type of data mobile carriers are capturing from their customers and how long that data is being retained. Among the data: who you called and when; who you texted and when; and text message content.

Just what personal information does your mobile carrier store about you, and for how long? A memo from the Department of Justice sheds some light on this question.

Obtained from the Department of Justice by the ACLU through the Freedom of Information Act, the memo entitled "Retention Periods of Major Cellular Service Providers" reveals the data retention policies of six major U.S. mobile carriers. Dated August 2010, the memo says it's directed for "Law Enforcement Use Only," so it was written specifically for police and legal authorities in cases where they need to obtain cell phone records.

As revealed in the memo, mobile carriers capture a variety of data about you, including call detail records (who you called and when), text message details (who you texted and when), text message content, bill copies, payment history, and even which cell towers you use. Another category, called Subscriber Information, presumably tracks your name and all necessary contact details.

But the carriers differ in which data they capture and for how long.

Verizon holds your call detail records and your text message details for 1 year, T-Mobile for 2 to 5 years, and AT&T for as long as 7 years. Verizon also keeps track of the cell towers you use for 1 year as does T-Mobile, while AT&T has been keeping that information since July 2008.

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The only two carriers who retain your text messages are Verizon (and only for 3 to 5 days) and Virgin Mobile (for 90 days, and a search warrant is required for access). And copies of your bills are kept by Verizon for 2 to 5 years and by AT&T for 5 to 7 years but are not retained at all by T-Mobile.

The ACLU sent a copy of the memo to Wired for its story yesterday, but it can also be found across the Web.

A spokesman for the ACLU told CNET that a "black and white photocopied version of the document (PDF) was sent to the ACLU of North Carolina as part of its FOIA request." Searching the Internet for the document, the ACLU found a couple of "clean" copies, "but no one ever noticed them or their importance until now."

Catherine Crump, staff attorney with the ACLU Project on Speech, Privacy, and Technology said in a statement that the group published the document because it feels this information should be public.

"People shouldn't be kept in the dark about what kinds of records are being kept on them, how long they're kept for, and who has access," Crump said. "The location information on cell towers used is especially troubling--few people realize that their every movement is being logged by their cell phone company, and that the government can get hold of it without a warrant. We hope that the more people know, the more likely it is that we can strengthen privacy protections."