No one in Claudine Anne Luera’s family knows when she first met the Border Patrol agent.

Her life had spiralled in recent years, as her heroin addiction consumed her. She lost custody of her five children. Not far from the house where she was born and raised in a south Texas border city, Luera, 42, seemed lost to those who knew her, trading sex to feed her addiction and living nowhere in particular, at friends’ houses and hotels.

One evening last week, her 17-year-old autistic son called her to share his news: he was now “mainstreaming” at school and part of regular high school classes. “She was excited,” said Luera’s sister, Colette Mireles, 37. “She did get emotional, but it sounded more of happiness.”

That was the last time Ms Mireles spoke to her sister. Luera’s body was found just a few days later, and the Border Patrol agent she had met shortly before her death – Juan David Ortiz, 35, a navy veteran who was a supervisory officer in the agency’s sprawling Laredo sector – was charged with killing her and three others.

In a case that has shocked a city where the ubiquitous green uniforms of the Border Patrol inspire both respect and fear, Ortiz has been accused of shooting and killing four people in a 12-day killing spree and dumping their bodies along roadsides. All of the victims, as well as a fifth person who managed to escape and helped the police capture him, were some of Laredo’s down-and-out – those who struggled with drug and alcohol addiction, and who had a history of run-ins with the law, law enforcement officials and relatives said.

“We’re thinking that maybe he was preying on them, picking them up frequently, probably thinking no one’s going to care for them, no one’s going to wonder if they go missing,” Ms Mireles said. “She didn’t deserve to die like this. She was loved by her family and her friends. She was someone. She was never forgotten.”

Law enforcement officials described Ortiz as a serial killer, but they acknowledged that they were still trying to unravel the mystery of a motive and why the killings came so swiftly in a matter of days. All four of the victims were women. Among them was a transgender woman identified by officials as Humberto Ortiz, 28, who also went by the name Janelle.

“Unfortunately, this is a community of people that are vulnerable,” Isidro Alaniz, the Webb County district attorney, told reporters. “Whether it be because of alcohol, substance abuse, drug addiction or prostitution, Ortiz is the one that decided who he was going to target.” He added, “It’s difficult to get into the mind of a killer. But what we have right now, and what we do know, is that Ortiz carried out these murders in a cold and callous way.”

Family members and friends of four women who authorities say were killed by a US Border Patrol agent gathered for a candlelight vigil at a park in Texas on Tuesday 18 September (AP)

In the end, Juan Ortiz appeared to have become suicidal, the authorities said. As officers were closing in, he ran to the third floor of a hotel parking garage and tried to use a parked pickup truck as a shield. He flashed his phone at the officers surrounding him, authorities said, and they concluded he was trying to make it appear that he was holding a weapon.

“Suicide by cop,” explained Federico Garza, the chief deputy for the Webb County sheriff’s office.

Ortiz was taken into custody without any shots fired, and subsequently admitted to the killings and the attack on the woman who escaped, authorities said.

Ortiz’s arrest has thrust Laredo into the national spotlight, exposed its underbelly and tested the relationship between residents and the thousands of Border Patrol agents who live and work in South Texas. In large swaths of the nearby brush, Border Patrol agents are some of the only visible symbols of the law, and in the urban heart of Laredo and other cities, president Donald Trump’s focus on border enforcement has given them expanded political clout.

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Officials said that they do not believe Ortiz attacked the women while he was on duty or used his government vehicle in committing the crimes. None of the victims were unauthorised immigrants; all appeared to be US citizens from the Laredo area. But authorities said they are looking at his service revolver as a possible murder weapon.

At least two of the bodies were found along stretches of Interstate 35, a major highway which cuts through Laredo and is one of the main traffic arteries along this part of the border. Ortiz knew every mile of Interstate 35 as part of the Border Patrol’s Highway Interdiction Team, which intercepts vehicles on the interstate suspected of engaging in drug trafficking and human smuggling.

In 2015, a federal appellate judge in the fifth US Circuit Court of Appeals described why the court found Ortiz’s testimony credible in a case involving a traffic stop he made on Interstate 35 in 2013. “Agent Ortiz has spent countless hours observing traffic patterns on Interstate 35, and has experience investigating human and narcotics trafficking,” the judge wrote.

On Monday, the street of stucco homes where Ortiz lived was quiet. No one answered at the home he shared with his wife and two children.

Ortiz served in the navy for eight years, joining the service two months before the 11 September attacks. He was never stationed outside the United States but spent time in medical training at various bases across the country. In June 2006, he obtained the rank of Hospital Corpsman second class.

Investigators believe four victims were fatally shot during separate attacks after the agent took each of them to desolate areas (AP)

He began working for the Border Patrol after leaving the navy in May 2009, and while he was an agent, earned a master’s degree from St. Mary’s University in San Antonio. He spent his entire career within the Laredo sector, which encompasses more than 100,000 square miles, spanning the border region in South Texas up north to the Texas-Oklahoma line.

Carla Provost, the recently named chief of the US Border Patrol, said Ortiz had been put on an unpaid indefinite suspension, and she defended the work of her agents, a handful of whom have made headlines in South Texas in recent years after being accused of murder, rape and other charges.

“In south Texas, I’ve got a workforce of approximately 6,000, and I do not want a couple of rogue individuals to characterise how my men and women work, and who they are, because they’re consummate professionals,” Ms Provost told reporters in Laredo.

At her home on Monday evening, Luera’s sister, Ms Mireles, pulled out family photos and laid them on a couch. She last saw her sister in May, when Luera was hospitalised with a leg infection. She watched as her sister struggled to cope without the heroin, and recalled that in the hospital she was shaking and vomiting.

Relatives at one point drove Luera to a rehab centre in Corpus Christi, in an attempt to get her off the drug. “Within two days, my sister was back in town,” Ms Mireles said.

Numerous Mexican-American families in Laredo have Border Patrol agents in the family. Luera’s family was no different. She said she was angry with Ortiz, not with the Border Patrol as a whole.

“These are men and women that go on the front line every day,” Ms Mireles said “We just want to let them know that we hold nothing, and we would never hold anything, against them.”