SALEM (AP) — Marion County officials have begun monitoring parolees and prisoners on supervised release with global positioning devices that resemble rubber wristwatches.

Worn like ankle bracelets, the devices alert authorities at the Marion County Sheriff’s Office when offenders violate the terms of their probation by stepping beyond set boundaries.

The devices, for example, trigger an alarm if felons go within 500 feet of their victims’ homes, said Lt. Sheila Lorance of Marion County.

Of the county’s roughly 4,400 offenders on supervised release, she said, about 25 to 35 felons are electronically monitored.

“It allows them to remain in the community,” she said. “The risk is that they can cut it off at any time.”

While the devices are programmed to alert county officials immediately if they’re removed by a parolee, it can take days for law enforcement officers to locate a missing felon.

Last month, Ronal William Collins, a high-risk violent sex offender on parole, removed his device.

Police issued an arrest warrant for Collins, 62, within an hour. But they did not find him until nine days later.

Most parolees who wear the monitors pay $10 per day to help cover costs, Lorance said, but the fee is waived for those who can’t afford it.

The county pays iSecuretrac of Omaha, Neb., $125,000 to oversee the devices’ transmissions. The company alerts deputies if offenders tamper with the device or violate their parole.

The company monitors GPS devices for law enforcement agencies in 42 states, said Chief Financial Officer Lincoln Zehr.

Recently released felons fitted with electronic monitors are still subject to parole and probation procedures, Lorance said. They must meet with parole officers regularly.