BOSTON -- The state began using global positioning satellite tracking technology to monitor some of the state's most dangerous sex offenders Wednesday.

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Pam Cross Reports On GPS Resources GPS For Sex Offenders?

NewsCenter 5's Pam Cross reported that some sex offenders still under state supervision will be required to wear ankle straps that can pinpoint their location instantly.

Beginning Wednesday, the state's most dangerous sex offenders who are released on parole will wear a GPS monitor so officers know where they are in the world.

"It's not 100 percent foolproof -- nothing is. But it is a tool that will let us know if a sex offender in this system is in the area of a crime," said state Sen. Steven Panagiotakos.

The million-dollar test program begins by putting monitors on 100 Level 3 sex offenders, the most dangerous. However, about 1,300 Level 3 offenders who have already been released cannot be forced to participate.

The technology was sparked by the murders last year of Joanne Presti and her daughter, Alyssa, in their Woburn home. A Level 3 sex offender registered in one city but spending time in their neighborhood has been charged.

"It's a very sad, sad case. We feel the system broke down," said Panagiotakos.

Exclusion zones, like schools, can be set up to let officials know if an offender has gone too far.

"A 'stay away zone.' For instance, around a victim's home -- if they penetrate that home within 500 meters, an alarm would go off and a monitoring center would be notified 24 hours a day," said Massachusetts Probation Department Deputy Commissioner Paul Lucci.

Opponents call satellite tracking an invasion of privacy for ex-convicts who have served their sentences. Proponents don't agree.

"But it's also exculpatory. If they're doing the right thing, we know right away they weren't involved in the crime," said Panagiotakos.

Although past sex offenders are not covered by the monitors, authorities called the move an important step to guard future security.