Anaheim police say Franc Cano and Steven Dean Gordon were both wearing GPS ankle bracelets when four women were assaulted and killed last fall and earlier this year.
Cano, 27, was convicted in 2008 for lewd and lascivious acts with a child under 14.
Gordon, 45, was convicted on two charges of lewd and lascivious acts with a child under 14 in 1992 and on kidnapping charges in 2002.
"You have Cano who is on state parole and federal probation," explained Anaheim Police Lt. Bob Dunn. "He's being monitored by state parole. You have Gordon, who is on federal probation. He has a federal probation officer that he reports in to."
And, as registered sex offenders and transients, both men were required to check in every 30 days with Anaheim police.
"As far as the Anaheim Police Department is concerned these two were compliant with their registration terms," said Dunn.
Dunn said Tuesday there are roughly 560 registered sex offenders in the city of Anaheim.
"But not all sexual offenders are monitored by ankle bracelet or are on parole or probation," said Dunn.
Dunn said Cano was wearing a state-issued ankle bracelet and Gordon was wearing a federal GPS device.
He said there were no requests from California or federal agencies to watch the two men more closely.
The California Department of Corrections supervises Cano and the U.S. Probation Office Central District of California supervises Gordon.
"Unfortunately, GPS monitoring cannot always deter crimes," said Luis Patino, spokesman for the California Department of Corrections and Rehabilitation. "It is a tool that shows us where a monitored offender has been and it can place them at the scene of a crime. A monitor has no way to detect whether a crime is being committed."
In an emailed statement, Patino said "GPS monitors are not designed to alert us when one sex offender comes into contact with another. Since offenders must often attend the same counseling classes, substance abuse treatment programs, or live in areas that are far from schools and parks, sex offenders often come into contact with each other."
Patino said the GPS monitors do alert officials "when offenders enter zones from which we have excluded them because potential victims may be in that vicinity."
"For example, exclusion zones are often set up to keep sex-offenders who have abused children from places where kids go," said Patino. "But other locations would not set off an alert. Determined criminals will go to great lengths to commit crimes and we cannot blame our crime-fighting tools for criminals’ actions.”
Bob Lotter is the founder of Newport Beach-based eAgency Mobile Security, which designs software products used by federal and state law enforcement agencies to catch sexual predators.
Lotter said GPS systems work best if used as "electronic fences" by creating specific or restricted geographic limitations.
"My guess is that they [Cano and Gordon] were not under an electronic fence, no one was actively monitoring where they were going," said Lotter. "It's just there as retroactive data."
Anaheim police did say the GPS anklet data was used to connect Cano and Gordon to the four women.
Lotter said his technology has been used in hundreds of arrests and convictions of child sexual predators.
"What worries me the most is that they (predators) can't stop themselves," said Lotter. " So I don't see a GPS bracelet deployed in the fashion that it was, and checking in every 30 days, a sufficient deterrent for people who have already proven that they can be dangerous."
Three women from Santa Ana were reported missing last fall: Kianna Jackson, 20, Josephine Vargas, 34, and Martha Anaya, 28.
Jarrae Nykkole Estepp, 21, was found in an Anaheim trash-sorting plant on March 14. Anaheim and Santa Ana police say her discovery connected the men to her killing and the disappearance of the three other women.
Police say they may know where the other three women are but didn't elaborate. Dunn said Tuesday there was no additional information on their possible locations.
The mother of Estepp fought back tears outside the Santa Ana courtroom where Cano and Gordon made a brief appearance after being charged Monday with four counts of special circumstances murder and four counts of rape.
"There's complete negligence all around," said Jodi Michelle Pier-Estepp. "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."
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 the movements of Cano and Gordon.
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.
Cano and Gordon each face four counts of special circumstances murder and four counts of rape.
They're scheduled to be arraigned on murder charges May 19 in Santa Ana.
Both men remain jailed without bail. If convicted, they could be sentenced to life without parole or the death penalty.
Orange County District Attorney Tony Rackauckas has not said if he intends to seek the death penalty in the case.
The Associated Press contributed to this report.