BOISE, Idaho - Nobody has broken out of the Idaho State Correctional Institution in more than 20 years. Prison officials like to think a hard-bitten corps of sentries with names like Cookie, Bongo and Chi Chi has had something to do with that.

The institution is the only state prison in the United States to use snarling, snapping sentry dogs to patrol its perimeter.

In a program begun in 1986, 24 mean dogs - mostly German shepherds, rottweilers and Belgian Malinois, with a few boxers and pit bulls - roam the space between the inner and outer chain-link fences 24 hours a day, ferociously defending their territory.

Get too close to the fence and they will bare their teeth, bark and lunge. Set foot in their space and they will attack.

The animals themselves are former death-row inmates - dogs that were deemed too dangerous to be pets and would have been destroyed at the local pound if they had not been given a reprieve and assigned to prison duty.

"We're basically giving them a second chance at a good, healthy life," said Corrections Officer Michael Amos, who heads the sentry dog program. "Those same instincts that make them a bad pet make them good sentries."

Prison officials say the canines save on manpower and are more reliable during power outages than electrical security systems and more effective in the fog and the dark than the humans posted in the lookout towers. They also seem to have a powerful deterrent effect.

No one has escaped from the 1,500-inmate medium-security prison since the dogs were brought in. No one has even tried to get past the fences since the early 1990s.

"The average offender has no problem engaging in a fight with a correctional officer - they're used to fighting with humans. But they don't want to mess with a 100-pound rottweiler who has an attitude and who wants to bite the snot out of them for climbing that fence," said James Closson, a dog trainer in Boise who arranged the donation of some overaggressive dogs to the prison when the sentry program was new.

Over the years, the dogs have bitten handlers, badly mauling a staff member who in the late 1990s entered the kennel without first making sure all the animals were caged. But no inmates locked up at the prison have been bitten, authorities said.

Dogs were once widely used as sentries in the United States, particularly after World War II, when canines that had been trained by the military were pressed into civilian service. The practice fell out of favor during the civil rights era as police dogs became associated with racist and repressive law enforcement, said Chris Byrne, owner of Stonehill Kennel and Unlimited Dogs, which provides police dogs to the New York Police Department.

Many prisons continue to use dogs for tracking escaped inmates or sniffing out drugs or other contraband, but not as sentries.

"Most facilities have gone to electronic motion detectors or electrical fencing," said Jay Christensen, deputy warden of security at the Idaho prison. "But technology can be circumvented. We had a guy at the Idaho Maximum Security Institution get through a motion detector system a few years back" by moving so slowly that the sensors didn't pick him up.

"The dogs are much more dependable and the cost is really low," he said.

In the early 1990s, three inmates at ISCI tried to escape through the one part of the fence that wasn't guarded by dogs at the time, Christensen said. The guards in the towers could not see them in the dark, but a dog along a nearby section of the fence sounded the alarm by barking.

The ruckus alerted the nearest tower guard, who fired a shot, hitting one of the convicts, Christensen said. The two others were so frightened by the shot that they gave up, and all three were recaptured, he said.

Officials promptly reconfigured the fence so that there were no sections without dogs, he said.

Angus Love, executive director of the Pennsylvania Institutional Law Project, an inmate advocacy group, said he knows of no complaints about the use of prison sentry dogs.

Adam Goldfarb, a spokesman for the Humane Society of the United States, said that the Idaho prison appeared to be handling the dogs well, but that he had mixed feelings about the program.