WASHINGTON (AP) — The Postal Service takes pictures of every piece of mail processed in the United States — 160 billion last year — and keeps them on hand for up to a month.

In an interview with The Associated Press, Postmaster General Patrick Donahoe said the photos of the exterior of mail pieces are used primarily for the sorting process, but they are available for law enforcement, if requested.

The photos have been used "a couple of times" by to trace letters in criminal cases, Donahoe told the AP on Thursday, most recently involving ricin-laced letters sent to President Barack Obama and New York Mayor Michael Bloomberg.

"We don’t snoop on customers," said Donahoe, adding that there’s no big database of the images because they are kept on nearly 200 machines at processing facilities across the country. Each machine retains only the images of the mail it processes.

"It’s done by machine, so there’s no central area where any of this information would be," he said. "It’s extremely expensive to keep pictures of billions of pieces of mail. So there’s no need for us to do that."

The images are generally stored for between a week and 30 days and then disposed of, he said. Keeping the images for those periods may be necessary to ensure delivery accuracy, for forwarding mail or making sure that the proper postage was paid, he said.

"Law enforcement has requested a couple of times if there’s any way we could figure out where something came from," he said. "And we’ve done a little bit of that in the ricin attacks."

The automated mail tracking program was created after the deadly anthrax attacks in 2001 so the Postal Service could more easily track hazardous substances and keep people safe, Donahoe said.

"We’ve got a process in place that pretty much outlines, in any specific facility, the path that mail goes through," he said. "So if anything ever happens, God forbid, we would be able very quickly to track back to see what building it was in, what machines it was on, that type of thing. That’s the intent of the whole program."

Processing machines take photographs so software can read the images to create a barcode that is stamped on the mail to show where and when it was processed, and where it will be delivered, Donahoe said.

The Mail Isolation Control and Tracking program was cited by the FBI on June 7 in an affidavit that was part of the investigation into who was behind threatening, ricin-tainted letters sent to Obama and Bloomberg. The program "photographs and captures an image of every piece of mail that is processed," the affidavit by an FBI agent said.

Mail from the same mailbox tends to get clumped together in the same batch, so that can help investigators track where a particular item was mailed from to possibly identify the sender.

"We’ve used (the Mail Isolation Control and Tracking program) to sort the mail for years," Donahoe said, "and when law enforcement asked us, ‘Hey, is there any way you can figure out where this came from?’ we were able to use that imaging."