JEFFERSON CITY, Mo. —  Technology that helps the military align targets and motorists find their way is being tapped to track some sex offenders forever. Spurred by headlines of released sex offenders accused of murder, some states are mandating use of the Global Positioning System (search) for tracking. Many lawmakers see electronic monitoring as a natural evolution of statutes that already require sex offenders to register their addresses with authorities.

At least four states — Florida, Missouri, Ohio and Oklahoma — passed laws this year requiring lifetime electronic monitoring for some sex offenders, even if their sentences would normally have expired. Similar bills have been proposed in Congress and other states, including North Dakota and Alabama, where lawmakers this week approved legislation and sent it to the governor.

But some civil-rights experts and defense attorneys contend such requirements are too onerous and attach the stigma and inconvenience of electronic anklets and GPS transmitters to those who may never commit a crime again.

GPS monitoring makes sense for a small group of high-risk offenders, evaluated case-by-case, said John La Fond (search), a law professor at the University of Missouri-Kansas City and author of the recently published book "Preventing Sexual Violence."

"A law that requires that everyone who has committed a crime against a young child should be subject to lifetime locator technology is simply foolish," La Fond said.

After a registered sex offender was charged in March with killing 9-year-old Jessica Lunsford, Florida legislators quickly mandated tougher prison sentences for people who commit sex offenses against children and required lifetime GPS monitoring after serving time.

Missouri Sen. Matt Bartle (search) liked the Florida legislation so much that he copied and expanded it to include repeat sex offenders who commit crimes — such as exposing oneself to a child — that would otherwise be punishable by seven years or less of imprisonment.

Bartle said he intentionally cast a wide net.

"I think the general public is really not terribly confident that we're getting it right when it comes to pedophiles — that this individual case-by-case approach is leading to some very horrific situations," he said.

A new Oklahoma law also requires habitual sex offenders to wear GPS monitoring devices for the rest of their lives. Ohio's budget funds lifetime GPS monitoring only for people classified as sexually violent predators.

Many other states use GPS monitoring for selected people on probation or parole but the monitoring ends with the sentence.

People on the tracking system must wear the electronic waterproof ankle bands at all times and stay within a certain distance from their separate GPS transmitters, which can be carried on belts, in purses or set down on desks and tables when at work or home. The transmitter is about 3 inches long and tall and 1 1/2 inches wide.

Part of what makes the technology attractive is the ability to trigger automatic alerts to law enforcement authorities — by e-mail, cell phone text messages or faxes — anytime sex offenders approach off-limits areas like a school or stray from their designated route between work and home.

Local authorities also have the ability to pinpoint a person's location at any moment — shown as a dot on a computer map that contains street names and the offender's traveling direction and speed.

Pro Tech Monitoring Inc. already uses GPS technology to follow about 5,000 people on parole, probation and house arrest in 38 states, said Richard Nimer, its vice president for business development. He said inquiries about offender tracking services spiked nearly fourfold after Lunsford's death.

The GPS technology is not foolproof, however.

Authorities in Boise, Idaho, say paroled child-sex offender William Lightner cut off a GPS bracelet and fled on July 23. Near Tallahassee, Fla., Kenneth Lamberton was wearing a GPS monitor awaiting a child-molestation trial when authorities allege he tried to force one girl into a sex act in March and another to expose herself in April.

Both men had been assigned passive GPS devices that send information once a day. Florida is switching to the active GPS devices, which instantly alert authorities to any violations.

Florida's experience shows offenders on GPS tracking are less likely to get in trouble than those under traditional supervision. The state Department of Corrections followed about 16,000 offenders placed on community supervision in the 2001-2002 fiscal year, including more than 1,000 under GPS monitoring.

Two years later, the department had revoked the community release of 31 percent of those on GPS monitoring, compared with 44 percent of those under traditional supervision. Nearly 6 percent of GPS-monitored offenders had committed new felonies or misdemeanors, compared with 11 percent of those who were not electronically monitored.

Lifetime electronic monitoring may be preferable to lifelong stints in mental-health institutions, said Jack King of the National Association of Criminal Defense Lawyers. But rather than targeting entire categories of criminals, states should mandate it after individual hearings, he said.

Kansas City civil-rights attorney Arthur Benson already is challenging Missouri's lifetime sexual offender registry.

"While these laws are often couched in terms of protecting the public against repeat offenses, at heart they are vengeful, punishing acts," Benson said.