Two parolees raped and killed at least four women while wearing GPS trackers, and there may be more victims, police have said.
Franc Cano and Steven Dean Gordon, both registered sex offenders, were wearing ankle bracelets when the women were assaulted and killed last autumn, Anaheim Police Chief Raul Quezada said at a news conference.
Authorities did not explain how Cano and Gordon allegedly managed to carry out the killings while under supervision, but Mr Quezada said data from the GPS devices "was one of the investigative tools we used to put the case together".
Police Lieutenant Bob Dunn earlier said the two men were complying with a requirement to check in monthly with authorities and police had no reason to watch them more closely and had not received any such request from other agencies.
The discovery of one woman's body on a conveyor belt at an Anaheim waste-sorting plant was the key to breaking the case, he said. Investigators are seeking the other bodies.
District attorney Tony Rackauckas told the news conference that investigators "put a stop to a serial killing that would likely have continued beyond this point".
Mr Quezada said detectives were confident that there was at least a fifth victim and perhaps more.
The department has contacted other places with missing persons cases across the country, Mr Dunn said.
Police believe the men killed a woman in Anaheim and three others in Santa Ana last October and November while on parole.
Cano, 27, and Gordon, 45, were arrested by investigators on Friday and they were charged yesterday with four felony counts of special circumstances murder and four felony counts of rape.
If convicted, they could face a minimum sentence of life without parole or the death penalty.
Police at first did not link the disappearances of the four women.
The string of disappearances in Santa Ana began in October after Kianna Jackson, 20, of Las Vegas, arrived in the city for a court hearing on four misdemeanour charges of prostitution and loitering to commit prostitution. Her mother said she stopped responding to her text messages soon after she arrived in Santa Ana.
Josephine Monique Vargas, 34, was last seen on October 24 after leaving a family birthday party to go to a store. The Times said Ms Vargas had a rough past that at times involved drug use and prostitution, but her mother said she had been trying to better her life.
Martha Anaya, 28, asked her boyfriend to pick up their five-year-old daughter so she could work on November 12, then stopped responding to his messages later that night. Police said she also had a history of prostitution.
In the weeks before she was found dead at the waste plant, Ms Estepp had become a regular on a strip of Beach Boulevard in Anaheim long known for prostitution.
"These individuals were not on our radar whatsoever," Santa Ana Police Chief Carlos Rojas said. "Our three missing in Santa Ana just completely went off the grid and we were trying to follow up as much as we could."
Santa Ana police searched a canyon, examined the women's mobile phone records, alerted hospitals, put the word out on social media and even checked motels they were known to frequent but without success.
Then the naked body of Jarrae Nykkole Estepp, 21, was found on March 14 on a conveyor belt at an Anaheim waste plant.
Once investigators concluded that Ms Estepp was killed and that she had "a similar profile to our victims, we were able to ... move forward", Mr Rojas said.
Police believe Cano and Gordon have known each other since cutting their ankle bracelets in 2012 and boarding a bus to Las Vegas using fake names. The men were arrested by federal agents on May 8, 2012, after a two-week stay at Circus Circus Hotel & Casino, according to documents filed in US District Court in Nevada.
Cano and Gordon had previously been ordered to register as sex offenders after being convicted in separate cases of lewd and lascivious acts with a child under 14.
Gordon was convicted in 1992 and also has a 2002 kidnapping conviction, according to the Orange County district attorney's office. Cano's conviction dates back to 2008, prosecutors said.
After fleeing Los Angeles in 2012, the two were rearrested and both pleaded guilty to failure to register as a sex offender. They were ordered to provide DNA samples and have their computers monitored by federal agents, according to the federal documents, which were first obtained by the Los Angeles Times.
The men also checked in with Anaheim police every 30 days, as required, and provided updated photos, fingerprints and addresses, Mr Dunn said.
Both men had checked in earlier this month, he added.
Cano was wearing a state-issued ankle bracelet and Gordon was wearing a federal GPS device, he said.