It was just after 10 p.m. when William Cotter, wearing a belt full of ammunition, burst into the home of his estranged wife, Dorothy, shooting her in the back with a sawed-off shot gun before taking his own life.

Just five days earlier, a court had ordered him to stay away from his wife after decades of drunken violence and she was carrying a panic button linked to the local police station, in Amesbury, Massachusetts. But it wasn't enough to save her on the night of March 26, 2002.

Fast-forward six years. Electronic surveillance technology is changing the way authorities in the United States monitor repeat offenders. Its advocates say the new technology could have saved Dorothy's life. Its detractors fear a widening breach of civil liberties and an illusory sense of protection.

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Coast to coast, authorities are expanding electronic monitoring to fight crime -- moving beyond its early use in tracking movements of sex offenders to include gang members who have been released on probation, people accused of repeated violence against women and even truant students at schools.

At the heart of the surveillance is a technology best-known for helping people on the road: the global positioning system.

Other countries are watching closely. GPS monitoring is already established in parts of Europe but applied more narrowly, and it's growing fast in Latin America, said Jeff Durski, spokesman for iSECUREtrac Corp, based in Omaha, Nebraska, which manufactures the devices and leases them to police and courts.

Massachusetts, one of the first states to employ it in 2006, now has about 700 people fitted with electronic bracelets that send signals via satellite to computer servers if they go places they shouldn't -- so-called "exclusion zones."

The Massachusetts law, which allows judges to impose electronic monitoring as a condition of a restraining order, has become a model for states such as Illinois and Oklahoma.

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