The new proposal "removes discretion from the Postal Inspection Service as to how the mail covers are implemented," Strickland said in an interview. "I worry quite a bit about the balance being struck here, and we're quite mystified as to how this got put in the legislation."

Officials on the Intelligence Committee said the legislation was intended to make the FBI the sole arbiter of when a mail cover should be conducted, after complaints that undue interference from postal inspectors had slowed operations.

"The FBI would be able to control its own investigations of terrorists and spies, and the postal service would have to comply with those requests," said an aide to the Intelligence Committee who is involved in the proposal but insisted on anonymity because the proposal remains confidential.

"The postmaster general shouldn't be able to substitute his judgment for that of the director of the FBI on national security matters," the aide said.

The proposal would generally prevent the post office from disclosing a mail cover. It would also require the Justice Department to report to Congress twice a year on the number of times the power had been used.

Civil rights advocates said they thought that the proposal went too far.

"Prison wardens may be able to monitor their prisoners' mail," said Lisa Graves, senior counsel for the American Civil Liberties Union, "but ordinary Americans shouldn't be treated as prisoners in their own country."

Marcia Hofmann, a lawyer for the Electronic Privacy Information Center, a public interest group here, said the proposal "certainly opens the door to abuse in our view."

"The Postal Service would be losing its ability to act as a check on the FBI's investigative powers," Hoffman said.

Postal officials refused to provide a tally of mail covers, saying the information was confidential. They said the Postal Service had not formally rejected any requests from the bureau in recent years.

A tally in 2000 said the Postal Service conducted 14,000 mail covers that year for a variety of law enforcement agencies, a sharp increase over the previous year.

The program has led to sporadic reports of abuse. In the mid-1970s the Church committee, a Senate panel that documented CIA abuses, faulted a program created in the 1950s in New York that used mail covers to trace and sometimes open mail going to the Soviet Union from the United States.

A suit brought in 1973 by a high school student in New Jersey, whose letter to the Socialist Workers Party was traced by the FBI as part of an investigation into the group, led to a rebuke from a federal judge, who found that the national security grounds for such mail covers were unconstitutionally vague.