Bruce Schneier, a computer security expert and an author, said whether it was a postal worker taking down information or a computer taking images, the program was still an invasion of privacy.

“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,” he said.

But law enforcement officials said mail covers and the automatic mail tracking program are invaluable, even in an era of smartphones and e-mail.

In a criminal complaint filed June 7 in Federal District Court for the Eastern District of 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.

In 2007, the F.B.I., the Internal Revenue Service and the local police in Charlotte, N.C., used information gleaned from the mail cover program to arrest Sallie Wamsley-Saxon and her husband, Donald, charging both with running a prostitution ring that took in $3 million over six years. Prosecutors said it was one of the largest and most successful such operations in the country. Investigators also used mail covers to help track banking activity and other businesses the couple operated under different names.

Other agencies, including the Drug Enforcement Administration and the Department of Health and Human Services, have used mail covers to track drug smugglers and Medicare fraud.

“It’s a treasure trove of information,” said James J. Wedick, a former F.B.I. agent who spent 34 years at the agency and who said he used mail covers in a number of investigations, including one that led to the prosecution of several elected officials in California on corruption charges. “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.”