SANTA ANA, Calif. (AP) -- GPS technology helped police link two convicted sex offenders to the rapes and killings of at least four women in California, but the mother of one victim said Tuesday that the monitoring system should have done more to prevent the crimes in the first place.
"If they were monitored correctly, then maybe none of this would have happened," said Jodi Michelle Pier-Estepp, the mother of victim Jarrae Nykkole Estepp, whose naked body was found March 14 on a conveyor belt at an Anaheim trash-sorting plant.
The situation has raised new questions about the effectiveness of the devices that are supposed to deter criminals by keeping them away from forbidden area such as schools and playgrounds and from anyone who has a protective order.
They are also supposed to be an investigative tool for law enforcement to track down convicts.
Federal and state officials said the devices worked as intended after Estepp, 21, was killed, pinpointing the locations of suspects Franc Cano, 27, and Steven Dean Gordon, 45, after the crimes.
"Unfortunately, GPS monitoring cannot always deter crimes," said Luis Patino, a spokesman for the California Department of Corrections and Rehabilitation, which was monitoring Cano. "They are tools that show us where a monitored offender has been and can place them at the scene of a crime. A monitor has no way to detect whether a crime is being committed."
Pier-Estepp disagreed as she fought back tears Tuesday outside the Santa Ana courtroom where Cano and Gordon made a brief appearance after being charged with four counts of special circumstances murder and four counts of rape.
The men are also accused of killing Kianna Jackson, 20, Josephine Monique Vargas, 34, and Martha Anaya, 28, who all went missing last fall in Santa Ana and had histories of prostitution.
In neighboring Anaheim, police Tuesday asked for public help to identify a potential fifth victim -- a tattooed women in her early 20s who went missing in February.
"There's complete negligence all around," Pier-Estepp said. "There's no excuse, no reason that the state can give me why these two men were even able to be around each other long enough to commit murder."
Darren Thompson, Cano's public defender, declined to comment. Gordon's attorney, Denise Gragg, did not return a call.
Orange County Senior Deputy District Attorney Larry Yellin said he had little information on whether the devices were used properly by state parole and federal probation agents who were monitoring Cano's and Gordon's movements.
Restrictions on how the devices are used are set by the agency overseeing the offender, Yellin said.
Electronic monitoring of criminals has become increasingly popular, with more than 100,000 tracked by via anklets nationally. But critics say the devices are far from a foolproof way to ensure that felons obey the law after being released from prison.
The monitors are not set up to alert authorities when two sex offenders are together, as Pier-Estepp suggested. That would be unworkable because sex offenders often attend the same counseling classes, substance abuse treatment programs, or live near each other in areas that are far from schools and parks to comply with state law, Patino said.
The devices send out multiple alerts a day, and it is up to often-overloaded parole and probation workers to sort out serious threats from glitches. In several cases, it took law enforcement days to notice criminals had tampered with their devices.
Last year, one Colorado parolee cut his bracelet but was not checked for several days, until he had fled and started a spree that claimed the lives of the state corrections chief and a pizza delivery man.
In New York state, audits showed federal probation agents ignored some warnings from a child porn suspect's bracelet for nine weeks, until he was arrested in the rape of a 10-year-old girl and the killing of her mother.
State and federal officials said they would not give details of how the devices were used to track Cano and Gordon, citing the ongoing investigation.
The pair had cut off their tracking bracelets in 2012, when they fled together under fake names to Las Vegas and stayed at the Circus Circus Hotel & Casino for two weeks before they were rearrested.
While Cano was still being tracked by state parole agents, Gordon was discharged from state parole in November and was being tracked for life by federal probation agents, officials said.
In November, Gordon petitioned a federal judge in Los Angeles to have his GPS bracelet removed, said Thom Mrozek, a spokesman for the U.S. attorney's office. Federal prosecutors successfully required Gordon to keep wearing the device.