The Mail Isolation Control and Tracking program was created after the anthrax attacks in late 2001 that killed five people, including two postal workers. Highly secret, it seeped into public view last month when the F.B.I. cited it in its investigation of ricin-laced letters sent to President Obama and Mayor Michael R. Bloomberg. It enables the Postal Service to retrace the path of mail at the request of law enforcement. No one disputes that it is sweeping.

“In the past, mail covers were used when you had a reason to suspect someone of a crime,” said Mark D. Rasch, who started a computer crimes unit in the fraud section of the criminal division of the Justice Department and worked on several fraud cases using mail covers. “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.’ Essentially you’ve added mail covers on millions of Americans.”