SACRAMENTO — Electronic monitoring was supposed to help Los Angeles County deal with the influx of thousands of felons moved out of California's prison system to ease overcrowding.

The nation's largest probation department strapped GPS ankle monitors on the highest-risk of those convicts, expecting the satellite receivers to keep tabs on where they spent their days and nights, and therefore keep the public safe.

Instead, agents are drowning in a flood of meaningless data, masking alarms that could signal real danger.

County probation officers are inundated with alerts, and at times received as many as 1,000 a day. Most of the warnings mean little: a blocked signal or low battery.

The messages are routinely ignored and at times have been deleted because there were so many, officers say.

Auditors making a spot check last fall found more than a dozen cases in which officers failed to notice that the devices were dead and probationers roamed unmonitored, some for weeks.

"If we keep getting false positives, we're not going to know the real one that means danger," said John Tuchek, a vice president for the Assn. of Probation Supervisors.

California's statewide system for monitoring sex offenders sends out as many as 40,000 alerts each month to state parole agents.

The consequences of ignoring such warnings can be disastrous.

In upstate New York, federal probation officers deluged with false alarms opted to disregard tampering alerts that cleared themselves within five minutes.

Because of that, no one noticed last year when a man facing child pornography charges broke the strap of his monitor and slapped it back together with duct tape. The man left the still-operational device at home, then traveled across town and raped a 10-year-old girl and stabbed her mother to death.

A U.S. District Court judge in New York released a report in April noting that probation officers in 12 of the nation's 94 federal court districts routinely ignored short-term alerts. Federal court officials ordered the practice stopped.

In Colorado last year, officers dismissed days of tampering and dead battery alerts from a parolee's GPS monitor. The man had slipped out of the device strapped to his ankle and killed a pizza delivery man and the state's corrections chief, authorities said. The fugitive was shot and killed days later while attempting to flee police in Texas.

Proponents of GPS technology say improving the system is a matter of better training, smaller caseloads and more effective technology to filter the flood of data.

Steve Logan, chief executive of Satellite Tracking of People, which monitors California sex offenders, said officials and the public should not see GPS tracking as a panacea.

Electronic monitoring is "a tool … not a silver bullet, but a really, really good tool," Logan said.

But some national GPS experts and parole officers say there are so many technological problems with GPS monitoring that it will never be as secure as officials promise.

"When these alerts are in the tens of thousands, it seems like an unwinnable situation," said Matthew DeMichele, a former researcher for the American Probation and Parole Assn. and coauthor of the Justice Department's guide on electronic monitoring.

"In some ways, GPS vendors are selling law enforcement agencies, politicians, the public a false bag of goods," he said.

The data overload disclosure comes as nearly every county in California is preparing for a massive expansion of the "virtual jail" — the use of GPS devices to track criminals on the street rather than incarcerate them.