The key to making the devices work, he and other experts said, is to figure out how best to process the immense amounts of information they generate.
The AP inquiry of correctional agencies found that policies on how to handle alerts vary. In Kentucky and Ohio, state probation or parole officers only respond to alerts during regular business hours. In other places, coverage exists around-the-clock.
Some agencies hire monitoring companies to do an initial screening of alarms. Others tackle that task themselves.
In many cases, alerts about things such as low batteries or lost GPS signals can be resolved by asking an offender to recharge or go to a window to recapture the satellite signal. Alerts that can't be resolved immediately are usually sent to field officers through pages, texts or emails. In most states, officials rarely respond in person because so many alerts can be cleared with a phone call.
While many agencies have detailed protocols in place, others have none.
In Florida, after a man on a GPS monitor was accused in an Easter Sunday shooting in the city of Apopka, administrators in the state's Ninth Judicial Circuit Court pointed to a "lack of procedures" for handling alarms.
In a June 7 report, court administrators said the small monitoring company that had been watching the suspect had just one person on call to track 81 people, and no system for notifying law enforcement or the courts about GPS violations. After the shooting, it took six hours for the company to notify anyone that the suspect had removed his tracker and disappeared.
Even when procedures are in place, they aren't always followed. For example, a 2010 audit of the federal probation office in northern New York found lapses that included officers not making their required number of in-person visits with people in monitoring programs.
A new review conducted after Renz cut off his bracelet found that even though his monitor had sent multiple tamper alerts over several weeks, officers neither inspected his equipment nor documented the alerts in his case file.
"In most instances, the probation office took no action in response to the alerts at all. When action was taken, it was limited to the probation officer verbally admonishing the defendant to `stop messing with the transmitter,' the report said.
In a June 14 letter to a New York congressman, Judge Thomas Hogan, director of the Administrative Office of the U.S. Courts, said Renz was not supervised in a typical manner, and that the office had since made substantial changes, including reorganizing the monitoring unit, retraining staff and "dismissing and demoting certain probation office personnel."
In Tennessee, a government audit last summer looked at a sample of 68 GPS offenders and found that officers with the state's Board of Probation and Parole had failed to clear or confirm 80 percent of the 11,347 alerts they generated over 10 months. That included thousands of alarms set off by people leaving home when they weren't supposed to or entering places they had been told to avoid.
The report also found that offenders placed on monitors because they were deemed to need extra supervision actually got less because corrections officials routinely skipped tasks such as verifying that sex offenders were attending mandatory counseling sessions.
In response, the state Department of Correction noted that officers in the tracking unit had an average caseload of 40 offenders -- more than the 25 suggested by the American Probation and Parole Association. It also noted that a center that had been screening alarms 24 hours a day was closed due to budget cuts in 2011, at a time when state lawmakers had more than doubled the number of sex offenders required to get bracelets.
In several states, agencies are making changes.
After the March slaying of Colorado Department of Corrections chief Tom Clements, the state began requiring parole officers to respond to tamper alerts within two hours. The suspect in Clements' death, Evan Ebel, was killed in a shootout with Texas authorities.
In 2011, California began requiring the companies that provide ankle bracelets to sort routine alerts from more urgent ones to help overwhelmed parole officers. Still, earlier this year state officials admitted that thousands of sex offenders had slipped their bracelets and become fugitives.
Legislators there are pushing for more serious punishments for removing a bracelet. Said Democratic state Sen. Ted Lieu: "Dangerous parolees do not cut off their GPS devices because they want to go to church unmonitored."
U.S. Rep. Dan Maffei, a Democrat who represents upstate New York, said he wants to make it a federal crime to tamper with a monitor.
Since the Apopka and Okafor cases, the chief judge of Florida's Ninth Judicial Circuit Court has suspended nearly all electronic monitoring of criminal defendants while the programs are reformulated.
In the Okafor case, an internal affairs report found that officers in the Orange County Corrections Department program weren't on duty to respond to alerts after 9 p.m., and staffing was so tight that disciplinary hearings weren't scheduled for offenders with violations. One supervisor told investigators the monitoring equipment, supplied by 3M, was issuing so many email alerts that it had caused confusion about which were legitimate.
Chris Defant, a technology manager with 3M, said some users of the technology are still working to refine their procedures, while others have designed their policies in ways that generate few alarms.
In Florida, Orange County spokesman Steve Triggs said the real problem was such monitoring programs were "never really designed for the Okafors of the world." Prosecutors had argued at a bail hearing that Okafor was too dangerous to be released. Okafor has pleaded not guilty to the home invasion and the shootings. His lawyer, Joseph Haynes Davis, said his client is innocent. Okafor is behind bars awaiting trial.