It sounded like something out of bad fiction: a cop asked to investigate murders that he himself had committed. It was 2019, and I was writing about a series of vicious killings that had taken place in the border city of Laredo a year earlier. Over a period of twelve days, two sex workers had been picked up from what were known as “the prostitute blocks” on San Bernardo Avenue, taken to remote areas northwest of the city, and shot in the head. After another potential victim made a daring escape from the killer’s white pickup truck, she led police to his house. Two more women would be shot and killed before he was apprehended. The man turned out to be a U.S. Border Patrol supervisor, Juan David Ortiz, who was part of the law enforcement team that had been assigned to hunt for the killer. This week, after more than five hours of deliberations, a jury found Ortiz guilty of capital murder; he was sentenced to life in prison without parole.

As I reported on the murders, everyone I spoke with who knew Ortiz, then 35, was flabbergasted. A former emergency medical technician for the U.S. Navy who had done a tour of duty in Iraq, Ortiz had joined the Border Patrol in 2009. One of his buddies from the Marines told me that “Doc”—that was Ortiz’s nickname—genuinely cared about the migrants crossing the border. “He wanted to use his medical skills to help migrants who had been traveling for days in the desert,” the Marine buddy said.

Ortiz was indeed a respected agent—dedicated and hardworking. There was only one complaint ever filed against him: a migrant accused Ortiz of stealing one of his cigarettes. But after an internal investigation, the complaint was dismissed. By 2017, Ortiz had been promoted to the position of “intelligence supervisor” at the South Texas Border Intelligence Center, in Laredo. In his spare time, he earned a master’s degree in international relations at St. Mary’s University, in San Antonio. He seemed destined for bigger and better things at the agency.

What’s more, people who knew Ortiz told me, he seemed to be such a good man. He and his wife, Daniella, were happily married. They and their two young children lived in a newly built, beige-colored stucco home. On Sundays, the family attended the First Assembly of God.

I did hear one story about Ortiz that perplexed me. One of his Marine buddies said that Ortiz would occasionally send him texts about the rigors of the job. He wrote that he felt like he was back in Iraq, going to war every day. The Marine buddy suspected Ortiz was experiencing long-repressed PTSD. He suggested Ortiz get some help. And Ortiz did, at least for a while, go to Laredo’s VA clinic, where he was prescribed medication.

Still, none of his coworkers sensed Ortiz had any sort of dark side. No one had ever heard him say anything about a hatred of sex workers. In fact, according to the Webb County district attorney, Isidro “Chilo” Alaniz, the lead prosecutor at Ortiz’s trial, just before the killings began in 2018, Ortiz was told he was being considered for another promotion at the South Texas Border Intelligence Center.

So why did such a seemingly decent, successful man suddenly begin killing women who worked as prostitutes? I was hoping that his trial, which began on December 1, would provide an answer.

Alaniz had the jury watch a video of Ortiz being interrogated for nearly ten hours by a Texas Ranger and a captain with the Webb County sheriff’s department. (It was the first time the interrogation had been made public.) For a few hours, Ortiz denied everything. Finally, in a matter-of-fact voice, he acknowledged he had done the killings. He said that when he drove along San Bernardo Avenue, “the monster came out” and urged him to “clean up” the avenue by getting rid of the prostitutes, whom he called “trash” and “so dirty.”

But Ortiz also openly acknowledged that he had been a customer of most of the women he killed. According to one woman who testified at the trial, Ortiz was a regular customer. She liked him. He was “nice, smart, funny, a normal guy.” He would give her money to buy narcotics, drive her to a drug house, and then have sex with her in his pickup on the side of a road, or at a park, or even at his home when his wife and children were out of town.

It didn’t make sense. On one hand, he enjoyed the company of these women. Then he wanted to kill them?

During his interrogation, Ortiz said that he did indeed suffer from PTSD from his deployment in Iraq and that he did take prescription medication for depression and anxiety. Sometimes, he said, when he downed the concoction of medications with alcohol, he would black out completely. It was during those blackouts, he seemed to be implying, that he unknowingly did the killings.

But district attorney Alaniz said Ortiz knew exactly what he was doing. He not only plotted out the killings, Alaniz claimed, he did his best to hide his behavior from his fellow investigators who were trying to hunt down the killer. At one point, Alaniz told me, one of the investigators called in a request to the South Texas Border Intelligence Center, asking for help in finding a veteran sex worker named Claudine Luera, who occasionally worked San Bernardo Avenue and who had told others that she had an idea about who was behind the killings. The next day, Luera was found murdered.

Alaniz asked the jury: Did Ortiz, who was on duty that day, hear about the call asking about Luera? Did he hunt her down and kill her before she got a chance to talk to the other investigators and perhaps identify Ortiz? “Or was it coincidence she died that day?” said Alaniz. “I don’t think it was.”

Following the verdict, family members of the victims were allowed to take the witness stand and speak to Ortiz. They told him that he was the personification of evil. Ortiz kept his head down, and when the trial was concluded, he was escorted out of the courtroom by bailiffs.

Alaniz told me that he thinks Ortiz’s murder spree will always remain a mystery. “There are times when you find out why someone committed a crime, and there are times when you don’t,” he said. “All I know is that I’m thankful Ortiz was caught. He could have gone a long time killing women and hiding the evidence that implicated him. He was a brutal, relentless killer, and he wasn’t going to let anyone stop him.”