The military is using the FBI to skirt legal restrictions on domestic surveillance to obtain private records of Americans' Internet service providers, financial institutions and telephone companies, the ACLU said Tuesday.

The American Civil Liberties Union based its conclusion on a review of more than 1,000 documents turned over by the Defense Department after it sued the agency last year for documents related to national security letters, or NSLs, investigative tools used to compel businesses to turn over customer information without a judge's order or grand jury subpoena.

"Newly unredacted documents released today reveal that the Department of Defense is using the FBI to circumvent legal limits on its own NSL power," said the ACLU, whose lawsuit was filed in Manhattan federal court.

ACLU lawyer Melissa Goodman said the documents the civil rights group studied "make us incredibly concerned." She said it would be understandable if the military relied on help from the FBI on joint investigations, but not when the FBI was not involved in a probe.

The FBI referred requests for comment Tuesday to the Defense Department. A department spokesman, Air Force Lt. Col. Patrick Ryder, said in an e-mail that the department had made "focused, limited and judicious" use of the letters since Congress extended the capability to investigatory entities other than the FBI in 2001.

He said the department had acted legally in using a necessary investigatory tool and noted that "unusual financial activity of people affiliated with DoD can be an indication of potential espionage or terrorist-related activity."

Ryder said the information in the ACLU claims came in part from an internal review of DoD's use of the letters.

"We have since developed training and provided it to the services for their use," he said.

He said that there was no law requiring it to track use of the letters but that the department had decided it was in its best interest to do so.

Goodman, a staff attorney with the ACLU National Security Project, said the military is allowed to demand financial and credit records in certain instances but does not have the authority to get e-mail and phone records or lists of Web sites that people have visited. That is the kind of information that the FBI can get by using a national security letter, she said.

"That's why we're particularly concerned. The DoD may be accessing the kinds of records they are not allowed to get," she said.

Goodman also noted that legal limits are placed on the Defense Department "because the military doing domestic investigations tends to make us leery."

In other allegations, the ACLU said:

- The Navy's use of the letters to demand domestic records has increased significantly since the Sept. 11 attacks.

- The military wrongly claimed its use of the letters was limited to investigating only Defense Department employees.

- The Defense Department has not kept track of how many national security letters the military issues or what information it obtained through the orders.

- The military provided misleading information to Congress and silenced letter recipients from speaking out about the records requests.

Goodman said Congress should provide stricter guidelines and meaningful oversight of how the military and FBI make national security letter requests.

"Any government agency's ability to demand these kinds of personal, financial or Internet records in the United States is an intrusive surveillance power," she said.