As part of its growing intelligence operations within the United States, the FBI has increased its surreptitious entry and search missions since the Sept. 11, 2001 attacks, according to an unclassified bureau document.
"The refocusing of FBI operational priorities and the new emphasis placed on intelligence-based activities...has resulted in a dramatic increase" in the demand for so-called "black bag" jobs, in which teams of highly-trained specialists covertly enter a home or office, search its contents and leave without indicating they had been there, states the budget document.
It does not detail how many of the secret searches it carries out, and the FBI did not respond to comment.
The bureau is asking Congress for an additional $5 million to pay for the operations and more than a dozen new specialized personnel.
In 1995, most of the FBI's secret search operations were related to criminal investigations, according to the document. Last year, close to 90 percent of such operations were for national security matters, it asserts.
"It's obviously troubling that people's homes are being searched, and they may never learn of it -- if they're never charged with a crime," said Lisa Graves of the Center for National Security Studies, a Washington, D.C.-based think tank which studies intelligence policy and constitutional issues. Graves told the Blotter on ABCNews.com she does not believe the searches receive sufficient judicial oversight.
The FBI did not respond to a request for comment.
In advance of a surreptitious search on a national security matter, the FBI is required to obtain approval from a secret body known as the Foreign Intelligence Surveillance Court. In a criminal matter, it must obtain a judicial warrant.
The FBI document obtained by ABCNews.com includes new details about how the secret search operations go down.
"The execution of covert entry/search operations usually requires the [FBI] to physically deploy a team of approximately 11 agent personnel full time over a period of time (usually at least 3 days) to the target location," the document states.
Last year, the unit's 18 agents spent an average of 97 days on the road, according to the document. That suggests the bureau carried out roughly 50 of the secret operations.
Last year, the operations were 90 percent successful, according to the document. But the team faces challenges from new advances in security technology and needs additional engineers, technicians and analysts to help plan and prepare for each operation.