Approximately 160 billion envelopes, packages and postcards were photographed by the United States Postal Service last year, reports The New York Times.
It was done as part of the Mail Isolation Control and Tracking program, according to The Times, in which Postal Service computers take pictures of the exterior of every piece of mail that passes through the system.
It's one of two programs The Times says shows that postal mail is under similar surveillance to phone calls and emails by the National Security Agency.
Letters and packages cannot be opened without a warrant. The tracking program reportedly only collects images of the outsides.
"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," James J. Wedick, a former FBI agent who spent 34 years at the agency, told The Times.
It is not known how long the government saves the images.
It appears to be a broad expansion of another postal surveillance program, called the mail covers program, which has been used for over a century.
Under the mail covers program, law enforcement officials can ask postal workers to record information from the outsides of letters or parcels before they are delivered to a particular person. The U.S.P.S. then passes this information on to the law enforcement agency that requested it.
Requests for the mail cover surveillance are granted for approximately 30 days, and can be extended for up to 120 days.
The New York Times reports that tens of thousands of pieces of mail each year are screened through the mail covers program.
Even without opening the mail, officials can obtain valuable information this way -- information that many would consider an invasion of privacy. Computer security expert Bruce Schneier tells The Times that the program can track "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."
Law enforcement officials claim these programs are essential to national security.
In one recent case, The Smoking Gun reports that by examining 60 pieces of mail from scanned images, the FBI was able to identify the suspect in the ricin letters sent to President Obama and New York City Mayor Michael Bloomberg in May of this year. A woman from Texas, Shannon Guess Richardson, was charged last month.