LOS ANGELES - A little more than a year ago, California began testing the GPS monitoring devices that track the movements of thousands of sex offenders.
The results were alarming.
Corrections officials found that the devices used in half the state were so inaccurate and unreliable that the public was "in imminent danger."
Batteries died early, cases cracked, and reported locations were off by as much as three miles. Officials also found that tampering alerts failed and offenders were able to disappear by covering the devices with foil, deploying illegal GPS jammers or ducking into cars or buildings.
The state abruptly ordered parole agents to remove every ankle monitor in use from north of Los Angeles to the Oregon border. In their place, they strapped on devices made by a different manufacturer - a mass migration that left California's criminal-tracking system not operational for several hours.
The test results provide a glimpse of the blind spots in electronic monitoring, even as those systems are promoted to law enforcement agencies as a safe alternative to incarceration. The flaws in the equipment raise the question of whether the state can deliver what Jessica's Law promised when voters approved it in 2006: round-the-clock tracking of serious sex offenders.
In a lawsuit over the state's GPS contracting, corrections attorneys persuaded a judge to seal information about the failures, arguing that test results could show criminals how to avoid being tracked and give parole violators grounds to appeal convictions.
The information, they warned, would "erode public trust" in electronic monitoring programs. The devices, they said, deter crime only if offenders believe their locations are being tracked every minute.
"The more reliable the devices are believed to be, the less likely a parolee may be to attempt to defeat the system," GPS program director Denise Milano wrote in a court statement.
State officials say the replacement devices have largely resolved the problems, but officials so far have refused to release test data showing what, if any, improvements were gained.
Through interviews and by comparing censored documents obtained from multiple sources, the Los Angeles Times was able to piece together most of what the state persuaded the courts to black out.
GPS tracking devices are designed to alert authorities if the wearer tampers with the device, tries to flee or strays too close to a school or other forbidden area. Currently, 7,900 high-risk California parolees and felons - most of them sex offenders or gang members - wear the devices strapped to their ankles.
A Times investigation in February found that thousands of child molesters, rapists and other high-risk parolees were removing or disarming their tracking devices - often with little risk of serving time for it because California's jails are too full to hold them.
The state's testing was conducted as part of a winner-take-all contest for the nation's largest electronic monitoring contract, worth more than $51 million over six years. Industry experts said they were the most exhaustive field trials they had seen.
When statewide monitoring began in 2008, California split the work between a division of 3M Co. and Houston-based Satellite Tracking of People, or STOP.
When California later sought to switch to a single provider, 3M came in with the low bid.
For a week in late 2011, parole agents abused both companies' devices. They were dropped 4 feet onto concrete, wrapped in foil to block their signals and submerged as long as three hours in a swimming pool. Testers allowed batteries to run dead, cut ankle straps and traveled into areas beyond the reach of satellite and cellular phone signals.
Officials declared 3M's devices so faulty that the state rejected the company's bid. A second round of tests showed 3M's ankle monitors posed a public-safety emergency, Milano said.
"The more reliable the devices are believed to be, the less likely a parolee may be to attempt to defeat the system."
California GPS program director