Convicted sex offender Steven Dean Gordon listens during opening statements at his trial in Santa Ana, Calif., in the deaths of four California women. (Sam Gangwer/Orange County Register/SCNG via AP)

The serial killers’ fourth known victim was found on the trash conveyor belt at a California recycling facility in March 2014, her naked body in such poor condition that those who discovered her could not tell how she died.

She had trauma to her genitals and trauma to her neck, where, distinctively written, she also had a tattoo of her mother’s name.

Investigators soon identified the young woman as Jarrae Estepp, a 21-year-old mother and Oklahoma native who had come to Orange County looking for sex work and instead, authorities say, met her death.

On Thursday, after just an hour of deliberating, a jury convicted one of her accused killers — registered sex offender Steven Dean Gordon — not just for her kidnapping and death, but for the murders of three other Santa Ana women who were reported missing in the fall of 2013.

Like Estepp, they were sex workers. Some were also mothers. None have been seen, dead or alive, since they disappeared three years ago.

Their cases initially seemed unrelated and stumped investigators, drawing criticism from the women’s family members who said police weren’t taking their concerns seriously, reported the Orange County Register. It wasn’t until the recycling plant workers found Estepp’s mangled body that law enforcement officials first recognized the common threads — and realized the four cases were possibly the work of a pair of calculating serial killers.

What broke open the case?

Data from the GPS monitors of two men, both registered sex offenders and apparent best friends, who authorities say wore the devices while committing their crimes. It was a critical clue that, however useful in these cases, brought intense scrutiny to the state and federal probation systems, including a lawsuit, internal reviews and proposed corrective legislation.

Critics wondered how two dangerous men supposedly under government monitoring could still have the freedom to commit such atrocities.

Gordon, 47, was the first to stand trial. He fired his defense attorney and represented himself, spending much of the proceedings admitting his involvement in the killings and disparaging his alleged accomplice and former best friend, 30-year-old Franc Cano, whose own criminal trial is expected to begin next year.

The men share a short but shady history.

In separate cases, the two were convicted of lewd and lascivious acts with a child under 14 and ordered to register as sex offenders — Gordon in 1992, Cano in 2008, reported the Associated Press. Gordon was also convicted of kidnapping in 2002.

Authorities believe the men have known each other since at least 2010, the year police arrested the duo in Alabama after Cano cut off his GPS device and fled there, reported the AP. Two years later, the men cut off their monitoring devices again before using fake names to board a Greyhound bus to Las Vegas. They were arrested by federal agents two weeks later and returned to California.

It was once again the men’s GPS monitors that led to their most recent arrests, not because they cut them off but because they left them on.

Steven Dean Gordon, right, of Anaheim, Calif., appears for his arraignment with associate defender Denise Gragg, left, during a hearing in superior court in Santa Ana, Calif., in 2014. (Mark Rightmire/Orange County Register via AP)

Police used a fingerprint found on a caulking gun near Estepp’s body to track down the man who threw it away, hoping he might have information on the woman, or at least be able to tell them where he threw out his trash, Deputy District Attorney Larry Yellin told a grand jury in 2014. If they could track the place where the young woman’s body was dumped, they might be able to find her killer.

The man who left the fingerprint directed investigators to a dumpster in an industrial park where, they later learned, Gordon worked and lived in an RV and where Cano often visited. An analysis of the GPS monitors of registered sex offenders in the area led detectives first to Cano and then, through cellphone records, to Gordon.

When data analysts compared the pair’s GPS coordinates, they found that both men appeared in the areas where the women disappeared and then soon after at the industrial park with the dumpster.

In April 2014, almost exactly a month after Estepp’s body was found, Gordon and Cano were arrested and charged with Estepp’s murder and rape and linked to the killings of three other women: Kianna Jackson, 20; Josephine Vargas, 34; and Martha Anaya, 28.

Anaheim police said at a news conference at the time that they believed the two men were also responsible for the death of a fifth, unidentified woman who was also still missing.

Both men were initially charged with rape, reported the AP, but prosecutors later dropped those charges against Gordon without explanation.

In a 13 hour interview with police in April 2014, Gordon told two versions of the killings — first he excluded Cano because he said he didn’t want to rat out his friend, then he offered a more full account. The details were specific and haunting, according to grand jury testimony, and he was able to identify the women by photo and match them with the timeline of their disappearances, enough for authorities to ascertain that he and Cano did kill them, authorities said.

Gordon said all four women were put in the industrial park dumpster, testimony shows. Their bodies were likely buried in trash at two separate landfills, and authorities decided after extensive research that excavation attempts would be complicated, time consuming and likely unsuccessful because of the amount of time that had passed since the women went missing.

“This is a cold, calculated, serial killing machine,” Yellin said in the grand jury hearing, “and but for the one body’s legs coming off the edge, Jarrae Estepp’s legs coming off the edge causing the workers to hit the stop and call 911, the machine would keep going.”

During his trial, Gordon told jurors he was “ultimately the one responsible” for the women’s deaths and scolded his parole and probation officers for not watching him more closely, reported the Los Angeles Times, a criticism deputy prosecutor Yellin called “white noise.”

“The probation department didn’t do it,” Yellin said of the killings, according to the L.A. Times. “The parole department didn’t do it.”

Yellin told jurors in his closing arguments that Gordon and Cano manipulated, raped and killed the women before taking great care to cover up their crimes, reported the L.A. Times. The men disabled their victims’ phones, used a car-wash apparatus at the industrial park to flush away any evidence and meticulously planned when to kill them — on days before trash was picked up.

It streamlined disposal, Yellin said.

“They are generating a very efficient killing and evidence-hiding machine,” Yellin told jurors, reported the L.A. Times. “They get more and more efficient.”

In his juror remarks, Gordon did not dispute the state’s version of events.

“Everything he said is 100 percent on target,” Gordon said, reported TV station Fox 11. “I even complimented him after you (the jurors) left. He brought his A game.”

Gordon added: “Our intention on these nights were beyond evil. No doubt about it.”

Yolanda Linder, left, and Jodi Estepp, right, aunt and mother of Jarrae Nykkole Estepp. (Sam Gangwer/Orange County Register via AP)

When the guilty verdicts were read, reported the AP, the victims’ family members trembled and wept.

“I can’t say it’s justice but it’s peace,” Jodi Estepp, Jarrae Estepp’s mother, told the AP. “It’s a little bit of peace.”

Her daughter fought her killers at the end, according to grand jury testimony. She sprayed Mace at them but couldn’t get away.

“She was a fighter, and she got them in the end,” Jarrae Estepp’s aunt, Yolanda Linder, told the Orange County Register. “They are off the streets and those monsters can’t hurt other families like they hurt us.”

Next week, Gordon and the jury will return for his sentencing hearing. He faces the death penalty or life in prison.

After the trial Thursday, Jodi Estepp told reporters that if there is a silver lining, it’s that the discovery of her daughter’s body brought closure for the families of the other victims.

“Jarrae’s death,” she said, “was not in vain.”

More from Morning Mix

The fentanyl crisis is so deadly in Canada that even funeral directors need the antidote

Dead at 99, Pearl Harbor’s ‘national treasure’: If you’ve visited the memorial, you know his face

A small Indiana town doesn’t have a police force because all its officers just quit