WATCH: Video Reveals How Actions of U.S. Border Officers Led to Death of 16-Year-Old Mexican Boy as Investigation Reveals History of Abuse, Intimidation and Impunity Along U.S.-Mexico Border

Cruz Velazquez was only 16 when a stranger handed down his death sentence.

In many respects, he was a typical teenager. He studied, rising to the 10th grade at the local high school. He worked, helping his grandmother run the small business that supported his family. And he looked after his younger sister, Reyna, who relied on him after their parents separated.

“He was kind of my dad,” Reyna said, “because since I was little he always helped me with homework, teach me sports, and everything he could.”

Tijuana, however, where Velazquez lived, is not a typical place. For decades, the city has been an important staging ground for the multi-billion-dollar business of moving illegal drugs from South and Central America into the United States. It is home to warring cartels who conduct business in bloodshed: there were 910 homicides in Tijuana in 2016, and with 785 homicides documented so far this year, 2017 is on pace to be one of the most violent years in recent memory.

“[Tijuana is] very important,” said Steve Gomez, a former FBI agent and ABC News consultant, who spent years working against the cartels. “[It’s] probably the most important city … for all of the cartels in Mexico.”

At the start of his presidential campaign, Donald Trump accused Mexico of exporting drugs and the crime that follows in their wake. He has since pledged to “build a wall” between the United States and Mexico and announced plans to deputize thousands of new U.S. Customs and Border Protection officers to police it.

“We have some bad hombres here,” said Trump of those who have been allowed to pass through the country’s southern border, “and we’re going to get them out.”

On the night of Nov. 18, 2013, Velazquez arrived the San Ysidro Port of Entry, the Western Hemisphere’s busiest land-border crossing. Government surveillance video obtained by ABC News of his encounter with border officers shows in gripping detail just how far officers would go as they questioned a young man caught between the powerful forces on opposite sides of the cross-border drug trade.

That footage — which aired for the first time Friday on Good Morning AmericaWorld News Tonight with David Muir20/20 and Nightline — sparked a year-long ABC News investigation of U.S. Customs and Border Protection with the non-profit Investigative Fund that revealed a history of cases in which the agency appeared to ignore accusations of mistreatment and abuse.

“There is a clear history of agents and officers engaging in what I believe was serious misconduct, documented by my office, in many instances who received little or no discipline whatsoever as a result,” said James Tomsheck, the former head of internal affairs for U.S. Customs and Border Protection, who has become a sharp critic of the agency after being ousted amid controversy.

In response to the investigation, the agency issued a statement, saying, “CBP takes all allegations of mistreatment seriously, and does not tolerate actions that are not consistent with our core values of vigilance, service to country and integrity.”

But for Velazquez, and for other young people, the consequences of that alleged misconduct were extreme, suggesting that at least some of the “bad hombres” are not the people crossing the border, but the people patrolling it.

Velazquez was playing with Reyna at his grandmother’s house when he abruptly announced that he was going to the gym.

“Why?” Reyna protested. “It’s too late.”

There was a movie on television that she was eager to watch with him.

No te preocupes,” he said. “Don’t worry. I’m gonna be back.”

He shouldered his bag and left — not for the gym, but for the border.

Velazquez was carrying two plastic bottles filled with liquid methamphetamine destined for the United States, where the highly-concentrated mixture could be boiled down to solid form and sold.

A dedicated student with dreams of higher education, Velazquez might have seemed an unlikely smuggler. Reyna said the signs were subtle, starting perhaps with a new group of friends.

According to former FBI agent Gomez, the cartels frequently enlist young drug mules from the city who are willing to embrace risky work for “a quick buck.” Its members can also manufacture motivation where it is lacking. The cartels gather information about the families of promising recruits, Gomez said, so that reluctant couriers might be reminded that their families’ safety depends on their success.

“That is part of the noose that they put around their neck,” Gomez said.

Velazquez was one of about 75,000 people to attempt to enter the United States through San Ysidro that day, and he would have expected an easier trip than most. He had a border crossing card allowing him to skip the long lines and enter the country through the so-called “Ready Lane.”

When he presented his credentials to the officer at the primary inspection area, however, he immediately ran into trouble. The officer was suspicious of Velazquez, so he sent him to a secondary checkpoint, a post manned by a pair of relatively new officers named Valerie Baird and Adrian Perallon, for further scrutiny.

They asked Velazquez to place his bag on the counter, and they removed the two bottles inside. Baird examined the bottles first, turning them over in her hands, and then passed them to Perallon. The officers would later say Velazquez claimed to be carrying only “juice,” but one bottle was labelled black tea, the other apple juice, and both of them appeared to contain the same viscous, yellow liquid.

At this point, Velazquez, and perhaps his family, were in danger. According to Gomez, the cartel would have had someone waiting on the other side of the border to confirm that he had crossed safely. If he was detained, his absence would have been reported immediately, potentially triggering a visit to his family back in Mexico.

“In his mind, the cartel is worse that the U.S. agents,” said Gomez. “A lot of times, they don’t even wanna hear the excuse. They’ll just kill everybody and make an example.”

There are conflicting accounts of exactly what happened next, but video of the encounter shows that Baird and Perallon did not use a nearby kit to test the contents of the bottles.

Instead, they appear to suggest, or at least permit, Velasquez to prove that the liquid was indeed juice. Baird pushed a bottle toward Velazquez and raised her right hand, suggesting he was led down a dangerous path with a familiar gesture.

Drink it.

Velazquez barely hesitated. He took a drink. Perallon, who maintains he always thought it was just juice, appears to tell him to take another sip. Velazquez complied.

While it is unclear whether Velazquez knew exactly what he was carrying, the officers had reason to be suspicious. In March, the agency issued a press release after a Juarez woman was caught attempting to smuggle 26 pounds of liquid methamphetamine through the border in bottles in El Paso.

Two minutes later, the officers appear to encourage Velazquez to drink more. He took two more swigs.

Baird and Perallon exchanged smiles.

In total, Velazquez swallowed four sips of a solution tests would later reveal to be more than a hundred times stronger than the typical dose of methamphetamine. According to Dr. Ben Nordstrom, chief clinical officer of the drug addiction treatment center Phoenix House, those drinks constituted a “truly massive overdose,” the biggest he had ever seen, but the effects would have been delayed as the drug was absorbed into his bloodstream.

“He’s not gonna be able to feel it,” Nordstrom said. “But his blood pressure is going to be creeping up all along. He’s gonna start to feel restless.”

The inspection continued as Velazquez’s temperature began to rise. It was a cool day, so Velazquez had pulled a white “Hollister” hoodie over his t-shirt and blue jeans, but suddenly he began to sweat. He wiped his brow. He pulled off his sweatshirt.

But the officers did not call for medical attention. Instead, they took him into custody. After an alert from a drug-sniffing dog, they put Velazquez in handcuffs and led him into a security office, where the effects of his overdose would become obvious. Not only was he sweating profusely, but he was shaking and struggling to breathe.

“I remember putting my hand on him and just told him to, like, relax, calm down,” Baird said later. “I thought he was nervous that he was going to get caught.”

Velazquez grew increasingly frantic and fearful as his temperature spiked and his heart raced.

“When I was standing with him, he was telling me to hit him. He wanted me to hit him,” Perallon added. “And then he just said that he didn’t want to die.”

He began to scream, prompting Nina Signorello, another officer who was stationed nearby, to come to his aid. Perallon was a fluent Spanish speaker, and Signorello knew enough of the language to pick up a few final fragments: “son quimicos” and “mi corazon” and “mi hermana.”

They were chemicals. My heart. My sister.

By the time paramedics arrived, Velazquez could no longer stand on his own. Officers loaded him onto a gurney. He was delirious, they said later. His eyes were rolling. He was thrashing so violently that they handcuffed him to the guardrails.

At one point, his temperature reached 105 degrees and his heart was beating 220 times per minute.

Velazquez lost consciousness in the ambulance on the way to the Sharp Medical Center in nearby Chula Vista, where doctors tried and failed to resuscitate him. Velazquez was pronounced dead, at age 16, less than a half hour after his arrival.

The autopsy report would rule his cause of death to be acute methamphetamine intoxication. The medical examiner determined “the manner of death is accident.”

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SOURCE: ABC News, Brian Ross, Brian Epstein, John Carlos Frey and Pete Madden