The U.S. Postal Service, an independent agency of the U.S. federal government, photographed the exterior of about 160 billion pieces of mail it processed last year under a secret surveillance program first introduced in the wake of anthrax attacks in 2001, according to a new report.

The New York Times' Ron Nixon reports on the USPS' "mail covers" and "Mail Isolation Control and Tracking program," which together give the government a "sweeping" view of the communications sent between its citizens.

He reports:

The mail covers program, used to monitor Mr. Pickering, is more than a century old but is still considered a powerful tool. At the request of law enforcement officials, postal workers record information from the outside of letters and parcels before they are delivered. (Actually opening the mail requires a warrant.) The information is sent to whatever law enforcement agency asked for it. Tens of thousands of pieces of mail each year undergo this scrutiny.

The Mail Isolation Control and Tracking program was created after the anthrax attacks in late 2001 that killed five people, including two postal workers. Highly secret, it seeped into public view last month when the F.B.I. cited it in its investigation of ricin-laced letters sent to President Obama and Mayor Michael R. Bloomberg. It enables the Postal Service to retroactively track mail correspondence at the request of law enforcement. No one disputes that it is sweeping.

"It is not known how long the government saves the images," Nixon adds.

As with the recent scandal over the widespread surveillance of telephone and digital communications by the U.S. National Security Administration, the key issue here is how the program has broadened considerably since its inception: a tool once used actively to investigate only those people suspected of committing a crime is now used more defensively, to record as much data as possible.

That information includes the names, addresses, return addresses and postmark locations of a piece of mail, according to the report. That gives the government "a pretty good map of your contacts," according to one of Nixon's sources, even if the agency isn't actually reading the contents of the envelope or package. 

A mail cover request can be granted for up to 120 days for the purpose of either "criminal activity" and "national security." Challenges to them have failed because courts do not consider there to be a reasonable expectation of privacy for the exterior of a piece of mail.