LOS ANGELES (AP) — Officers in the nation’s largest probation department are being bombarded with alerts from GPS ankle monitors, potentially masking true alarms of danger.

The Los Angeles Times reported Sunday that Los Angeles County probation officers routinely ignore or even delete messages from these monitors because there are so many.

The GPS anklets are strapped onto the highest-risk convicts. Officers have received as many as 1,000 a day  with many warning of a blocked signal or low battery.

Auditors last fall found more than a dozen cases where officers didn’t notice devices were dead and probationers were roaming unmonitored, in some cases for weeks.

The issue has led to disastrous consequences elsewhere.

In upstate New York, federal probation officers overwhelmed with false alarms ignored tampering alerts and didn’t notice when a man facing child pornography charges broke his monitor strap, taped it together with duct tape, and left it still-working at home. The man 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 that said probation officers in 12 of the nation’s 94 federal court districts routine ignored short-term alerts. Officials ordered the practice stopped.

In Colorado last year, officers ignored days of tampering and dead battery alerts from a parolee’s anklet to find the man had slipped out of the device and killed a pizza delivery man and the state’s corrections chief. He was shot and killed days later while trying to flee police in Texas.

California began using GPS tracking in 2008 after voters passed Jessica’s Law, which required round-the-clock monitoring of serious sex offenders. More than 8,000 state parolees are tracked under that law.

Counties have expanded their use of anklets to track criminals on the street rather than in lock up after a federal court order requiring California to reduce prison overcrowding. More than 10,000 state inmates and parolees have been shifted to local control in the last two years.

GPS anklet supporters say improving the system requires better training, smaller caseloads and tools to better filter the data.

The ankle monitors track felons via satellite, transmitting their location over cellular networks to a central computer. When an offender removes their device or enters a prohibited area such as a school or park, an alert goes out to the supervising officer.

But alerts are also triggered for GPS signal loss, for example when tall buildings block the satellite; run-down batteries; cracked cases and loose straps.

Field tests by California corrections officials in 2011 showed that the devices used to track nearly half of the state’s sex offenders reported no signals 55 percent of the time.