"Basically they are doing the same thing as the other programs, collecting the information on the outside of your mail -- the metadata, if you will -- of names, addresses, return addresses and postmark locations, which gives the government a pretty good map of your contacts, even if they aren't reading the contents," computer security expert Bruce Schneier told The New York Times.
The warrantless surveillance program for law enforcement is called Mail Isolation Control and Tracking. It's been in effect almost 11 years.
Last year alone Postal Service computers photographed the outside of about 160 billion letters and parcels, the Times said.
It is not known how long the government keeps the images, the newspaper said.
The images are kept as part of a law enforcement surveillance technique known as "mail covers."
Mail covers do not involve reading the mail itself, just the outside information, and are not considered by the Postal Service or Justice Department to be constitutional violations since the outside of envelopes and packages can readily be seen by anyone.
"It's a treasure trove of information," former FBI agent James Wedick told the Times.
"Looking at just the outside of letters and other mail, I can see who you bank with, who you communicate with -- all kinds of useful information that gives investigators leads that they can then follow up on with a subpoena," said Wedick, who spent 34 years at the FBI and used mail covers in a number of investigations.
But the program's sweeping nature disturbs some people.
"In the past, mail covers were used when you had a reason to suspect someone of a crime," Mark Rasch, who started a computer crimes unit in the fraud section of the Justice Department's criminal division, told the Times.
"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,'" he said. "Essentially you've added mail covers on millions of Americans."
The Mail Isolation Control and Tracking program was created after the anthrax attacks in late 2001 that killed five people, including two postal workers, and infected 17 others.
The FBI said in a criminal complaint June 7 a postal investigator tracing the ricin letters was able to narrow the search to New Boston, Texas, actress Shannon Guess Richardson by examining front and back images of 60 pieces of mail scanned just before and after the tainted letters were sent to Obama and Bloomberg.
The data showed return addresses near her home, the FBI said.
Richardson originally accused her husband, Nathan Richardson, of mailing the letters, but investigators determined he was at work when the letters were mailed.
Shannon Richardson was indicted and charged Friday in the mailing of the letters. She was also charged with sending a third ricin letter to Mark Glaze, the executive director of Mayors Against Illegal Guns.