The US Postal Service records the outside of every piece of snail mail processed in the country, allowing employees to retroactively track correspondence at the request of law enforcement and national security agents, according to a published report.

The Mail Isolation Control and Tracking program is more than a decade old, but it came into public view recently when the FBI cited it in its investigation of ricin-laced letters sent to President Obama and New York Mayor Bloomberg. The highly secretive program uses computers to photograph the exterior of every piece of paper mail processed in the US—totaling about 160 billion pieces last year—according to an article published Wednesday by The New York Times. It was created after anthrax letter attacks in late 2001 killed five people, including two postal inspectors.

The program is the lower-tech companion to the formerly top secret surveillance program that collects the records of all phone calls and e-mails processed by US firms. Details including the time the communication was made, the initiating location, and who the sender and receiver were are indiscriminately gathered by the National Security Agency, the revelations later showed.

Government attorneys have long argued that programs inspecting the outside contents of snail mail are legal because there's no reasonable expectation of privacy for things printed on an envelope. Defenders of the more recent phone and e-mail records collection program have made similar arguments concerning the "metadata" related to electronic communications. But critics warn that the mail inspection program exposed by the NYT has the potential to go too far.

"In the past, mail covers were used when you had a reason to suspect someone of a crime,” Mark D. Rasch, the former director of the Justice Department's computer crime unit, told the NYT. Rasch himself has worked on several fraud cases using mail covers. "Now it seems to be 'Let’s record everyone’s mail so in the future we might go back and see who you were communicating with.' Essentially you’ve added mail covers on millions of Americans."

While investigators must get a warrant to read the inside of a letter, obtaining detailed exterior information collected under an even older "mail covers" program requires that only a form be filed. A judge's order isn't even necessary.