Washington ? They have broken into homes, offices, hotel rooms and automobiles. Copied private computer files. Installed hidden cameras. Listened with microphones in one couple’s bedroom for more than a year. Rummaged through luggage. Eavesdropped on telephone conversations.

It’s the FBI, operating with permission from a secretive U.S. court in a high-stakes effort pitting the nation’s premier law enforcement agency against the world’s spies and terrorists.

Most Americans never see this side of the FBI.

“The average citizen has no idea whether information about them might be caught up in one of these investigations,” said David Sobel of the Washington-based Electronic Privacy Information Center, an expert on this type of surveillance.

The Foreign Intelligence Surveillance Act ” enacted in 1978 and strengthened after Sept. 11, 2001, by the Patriot Act ” gives investigators a potent arsenal against “agents of a foreign power.”

The Bush administration this week won an important court victory affirming its plans to expand these tactics to more cases.

Besides break-ins, agents have pried into safe deposit boxes, watched from afar with video cameras and binoculars and intercepted e-mails. They have planted microphones, computer bugs and other high-tech tracking devices.

“The whole thing is very, very mysterious and quiet,” said Plato Cacheris, the Washington lawyer who represented spies Aldrich Ames and Robert Hanssen. “There’s not a lot that anyone can tell you.”

Surveillance is a deadly serious game among the trench-coat set featuring “black bag” jobs and wiretaps. Their 007-like gadgets ” one captures every keystroke typed on a target’s computer ” and the specialized agents who use them are among the best available.

These tools and the law are “designed to target intelligence officers and people trained by intelligence officers,” said Michael Woods, a former senior FBI lawyer who coordinated many investigations.

Nearly all those known to have been targeted did not detect what was happening until FBI agents flashed guns and badges.

The bureau is cautious. Agents never broke into Hanssen’s home in the Washington suburbs because they couldn’t find time when his wife or children weren’t there, according to people familiar with the case.

“They’re very good at not getting found out,” said Nina Ginsberg, a lawyer in Alexandria, Va., who has represented three people under surveillance. “I’m sure they would sit outside a house for a week before they made sure they could go in.”

The FBI watched Therese Marie Squillacote, a Defense Department lawyer, and her husband for 18 months. Over that time, they broke into their home three times and planted a microphone in their bedroom to monitor their conversations, according to court records. She was sentenced in 1999 to nearly 22 years for attempting to spy for East Germany and Russia with her husband.

What little is known about these FBI techniques emerges from court records spread across dozens of cases. But only a fraction of these nearly 1,000 surveillances each year result in any kind of public disclosure, so little is known outside classified circles about how they work.