Franc Cano and Steven Dean Gordon, both registered sex offenders, were both wearing ankle bracelets when the women were raped and killed last fall, Anaheim police Chief Raul Quezada said at a news conference.
The discovery of one woman's body on a conveyor belt at an Anaheim trash-sorting plant was the key to breaking the case, he said. Investigators were seeking the other bodies.
"They put a stop to a serial killing that would likely have continued beyond this point," District Attorney Tony Rackauckas said at the news conference.
Quezada said authorities 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, Anaheim police Lt. Bob Dunn said earlier.
Cano, 27, and Gordon, 45, were arrested by investigators on Saturday. Each man was charged Monday 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. They were being held without bail and expected to be arraigned Tuesday.
Police believe the men killed the woman last October and November while on parole.
Police believe Cano and Gordon have known each other since cutting their ankle bracelets in 2012 and boarding a Greyhound 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 U.S. District Court in Nevada.
Both were wanted fugitives: Gordon traveled using the alias Dexter McCoy and Cano chose Joseph Madrid, authorities said.
The investigation into the killings started after the naked body of Jarrae Nykkole Estepp, 21, was found March 14 on the conveyor belt. Authorities linked the suspects to her slaying, and to the disappearances last fall of three women who frequented a Santa Ana neighborhood known for drug dealing and prostitution.
Cano and Gordon were previously 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 re-arrested 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, Dunn said.
Police have not provided details on what led them to arrest the men for the killings or how they could have committed the crimes while under police supervision. Because the two were in compliance with their monthly check-ins, police had no reason to watch them more closely, Dunn said, and they had not received any requests from other agencies to do so, he said.
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 misdemeanor 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 Oct. 24 after leaving a family birthday party to go to a store. The Times said 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 5-year-old daughter so she could work on Nov. 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, Estepp had become a regular on a strip of Beach Boulevard in Anaheim long known for prostitution.