Another system, the Mail Isolation Control and Tracking Program, was created after anthrax attacks killed five people, including two postal workers, in late 2001. It is used to track or investigate packages or letters suspected of containing biohazards like anthrax or ricin. The program was first made public in 2013 in the course of an investigation into ricin-laced letters mailed to President Obama and Michael R. Bloomberg, then New York City’s mayor, by an actress, Shannon Guess Richardson.

Despite the sweep of the programs, postal officials say they are both less intrusive than that of the National Security Agency’s vast collection of phone and Internet records and have safeguards to protect the privacy and civil liberties of Americans.

“You can’t just get a mail cover to go on a fishing expedition,” said Paul J. Krenn, a spokesman for the Postal Inspection Service. “There has to be a legitimate law enforcement reason, and the mail cover can’t be the sole tool.”

The mail cover surveillance requests cut across all levels of government — from global intelligence investigations by the United States Army Criminal Investigations Command, which requested 500 mail covers from 2001 through 2012, to state-level criminal inquiries by the Georgia Bureau of Investigation, which requested 69 mail covers in the same period. The Department of Veterans Affairs requested 305, and the State Department’s Bureau of Diplomatic Security asked for 256. The information was provided to The Times under the Freedom of Information request.

Postal officials did not say how many requests came from agencies in charge of national security — including the F.B.I., the Department of Homeland Security and Customs and Border Protection — because release of the information, wrote Kimberly Williams, a public records analyst for the Postal Inspection Service, “would reveal techniques and procedures for law enforcement or prosecutions.”

Defense lawyers say the secrecy concerning the surveillance makes it hard to track abuses in the program because most people are not aware they are being monitored. But there have been a few cases in which the program appears to have been abused by law enforcement officials.

In Arizona in 2011, Mary Rose Wilcox, a Maricopa County supervisor, discovered that her mail was being monitored by the county’s sheriff, Joe Arpaio. Ms. Wilcox had been a frequent critic of Mr. Arpaio, objecting to what she considered the targeting of Hispanics in his immigration sweeps.