Similarly, John Radsan, an assistant general counsel at the C.I.A. from 2002 to 2004 and now a law professor at William Mitchell College of Law in St. Paul, said, “The C.I.A. is not supposed to have any law enforcement powers, or internal security functions, so if they’ve been issuing their own national security letters, they better be able to explain how they don’t cross the line.”

The Pentagon’s expanded intelligence-gathering role, in particular, has created occasional conflicts with other federal agencies. Pentagon efforts to post American military officers at embassies overseas to gather intelligence for counterterrorism operations or future war plans has rankled some State Department and C.I.A. officials, who see the military teams as duplicating and potentially interfering with the intelligence agency.

In the United States, the Federal Bureau of Investigation has complained about military officials dealing directly with local police — rather than through the bureau — for assistance in responding to possible terrorist threats against a military base. F.B.I. officials say the threats have often turned out to be uncorroborated and, at times, have stirred needless anxiety.

The military’s frequent use of national security letters has sometimes caused concerns from the businesses receiving them, a counterterrorism official said. Lawyers at financial institutions, which routinely provide records to the F.B.I. in law enforcement investigations, have contacted bureau officials to say they were confused by the scope of the military’s requests and whether they were obligated to turn the records over, the official said.

Companies are not eager to turn over sensitive financial data about customers to the government, the official said, “so the more this is done, and the more poorly it’s done, the more pushback there is for the F.B.I.”

The bureau has frequently relied on the letters in recent years to gather telephone and Internet logs, financial information and other records in terrorism investigations, serving more than 9,000 letters in 2005, according to a Justice Department tally. As an investigative tool, the letters present relatively few hurdles; they can be authorized by supervisors rather than a court. Passage of the Patriot Act in October 2001 lowered the standard for issuing the letters, requiring only that the documents sought be “relevant” to an investigation and allowing records requests for more peripheral figures, not just targets of an inquiry.

Some Democrats have accused the F.B.I. of using the letters for fishing expeditions, and the American Civil Liberties Union won court challenges in two cases, one for library records in Connecticut and the other for Internet records in Manhattan. Concerned about possible abuses, Congress imposed new safeguards in extending the Patriot Act last year, in part by making clear that recipients of national security letters could contact a lawyer and seek court review. Congress also directed the Justice Department inspector general to study the F.B.I.’s use of the letters, a review that is continuing.