As the world focuses on the high-tech spying of the National Security Agency, the misplaced card offers a rare glimpse inside the seemingly low-tech but prevalent snooping of the United States Postal Service.

Mr. Pickering was targeted by a longtime surveillance system called mail covers, but that is only a forerunner of a vastly more expansive effort, the Mail Isolation Control and Tracking program, in which Postal Service computers photograph the exterior of every piece of paper mail that is processed in the United States — about 160 billion pieces last year. It is not known how long the government saves the images.

Together, the two programs show that snail mail is subject to the same kind of scrutiny that the National Security Agency has given to telephone calls and e-mail…

In a criminal complaint filed June 7 in Federal District Court in Eastern Texas, the F.B.I. said a postal investigator tracing the ricin letters was able to narrow the search to Shannon Guess Richardson, an actress in New Boston, Tex., by examining information from the front and back images of 60 pieces of mail scanned immediately before and after the tainted letters sent to Mr. Obama and Mr. Bloomberg showing return addresses near her home. Ms. Richardson had originally accused her husband of mailing the letters, but investigators determined that he was at work during the time they were mailed.