Gustavo Ángel Suárez Castillo, an American citizen from San Antonio, piled six friends, including two brothers, into his white pickup truck with Texas plates just before dawn, having spent the night celebrating the news that he was going to be a father. Suddenly, four vehicles filled with armed men began chasing and firing at them.

The pickup truck crashed and as the passengers tumbled out, the armed men threw some to the ground, shooting one in the back, survivors told The New York Times. One recounted how he watched his brother slowly stop breathing while the assailants blocked medics from arriving.

When it ended, five of the men,  including Mr.  Suárez, were dead and the other two severely injured.

The attackers? Uniformed Mexican soldiers.

The shooting in the city of Nuevo Laredo in the early hours of Feb. 26 has been called a coldblooded execution by the survivors and a top government official. So far, four of the 21 soldiers involved in the encounter have been arrested and the case is under investigation by civilian prosecutors and the military.

The episode has deepened concerns about the growing footprint of Mexico’s armed forces, which has not only been put in charge of domestic security, but has also been given a rapidly expanding portfolio of businesses, like a new international airport and a major rail line.

It underscores what human rights advocates and analysts say is a dangerous flaw in Mexico’s governing system: one of the country’s most powerful institution operates with little oversight.

Despite a long history of human rights abuses, the military assumed responsibility for civilian security after the federal police was dissolved in 2019, taking on the country’s violent criminal organizations, but also putting residents at risk of becoming victims of heavy-handed tactics, critics say.

The Defense Ministry is under the command of an active-duty general, not a civilian leader, is not required to publicly release documents or report on its activities and often refuses to appear before Mexico’s Congress to answer questions.

The military’s strict control over its affairs has led the Mexican president to consolidate government projects under the armed forces to limit their transparency and has meant that cases of civilian deaths at the hands of the army almost never go to trial.

“Given the increasing role of the armed forces in Mexico, it is really crucial and urgent” that they “are regulated with a civilian supervision mechanism, which should be created to control and eventually get accountability,” said Marta Hurtado, a spokeswoman for the United Nations High Commissioner for Human Rights.

The U.N. has called for an independent investigation into the Nuevo Laredo killings, citing the military’s history of excessive use of force in the city.

An initial military statement implied that the men in the pickup were armed and had not heeded orders from soldiers.

But that claim was contradicted by Alejandro Encinas, a top federal government human rights official.

“It was not a confrontation,’’ Mr. Encinas said. “They were executed.”

The soldiers fired 117 rounds during the incident even though the victims never brandished a weapon, a preliminary report by the National Commission on Human Rights found.

The Defense Ministry declined to comment on the killings, citing the ongoing investigations.

Asked for comment on Mr. Suárez’s killing, an American official said the U.S.  government had issued its highest-level warning for Tamaulipas, the state that includes Nuevo Laredo, and had warned its citizens not to travel there.

Lawyers representing the families of the dead and the survivors say the army has tried to cover up details of what unfolded that morning.

They accuse the soldiers of removing the truck’s license plates to bolster their accusation that the men were behaving suspiciously. A survivor said he was forced at gunpoint to tape a confession that the men had fired on the soldiers first.

A week after the attack, about a dozen soldiers showed up around midnight at one of the survivor’s homes in an attempt to intimidate him into silence, his lawyers say.

“We do not understand why they shot some young people who were not even attacking them,’’ said Raymundo Ramos, the president of the Committee of Human Rights in Tamaulipas, an advocacy group representing the survivors and the families of the dead men.

(An earlier New York Times investigation revealed that Mr. Ramos had been spied on illegally by the military while working on a different case in Nuevo Laredo involving the armed forces and accusations of human rights violations.)

During the administration of Mexico’s president, Andrés Manuel López Obrador, the military has moved well beyond its main enforcement and security mission and into a variety of lucrative businesses.

It built and operates Mexico City’s new airport and is constructing much of the nation’s largest tourism project, a $20 billion, nearly 1,000 mile railroad that it will also manage once completed. The armed forces are also now in charge of the country’s customs, one of Mexico’s biggest income generators, with expected revenues of $59 billion for 2022.

Such responsibilities, analysts warn, give the military the ability to raise money on its own and could undermine Mexico’s balance of power.

At the same time, in Nuevo Laredo, just across the border from Texas, the military’s long track record of abuses has bred deep resentment.

Mr. Ramos’ organization has documented 18 cases of human rights violations linked to the military since 2018, including executions, rape and torture of civilians. But only one has gone to trial.

In one case, a 4-year-old girl, Heydi Mariana, was shot and killed last August when the car she was riding in came under fire from soldiers. At least 16 bullets ripped through the vehicle.

The military said the girl was killed during a confrontation with criminals, but has not provided evidence. No one has been charged in the case.

“My daughter was going to kindergarten,’’ said the girl’s mother, Cristina Rodríguez, 26, who added that soldiers showed up at Heydi’s funeral, a move the family interpreted as an act of intimidation. “She was not a delinquent.’’

The night before the February attack on the pickup truck, the victims, all in their 20s, were at a local nightclub to toast the news that Mr. Suárez was going to be a father.

After piling into Mr. Suárez’s truck, they passed four military vehicles carrying 21 soldiers that started pursuing the men. In a statement, the military said the soldiers had heard gunshots from the direction of the pickup.

The account of what happened next is based on interviews with the survivors, relatives of those killed, their lawyers and the government’s report.

The soldiers rammed one of their vehicles into the truck without identifying themselves or asking them to stop, the survivors say, a statement confirmed by the National Commission for Human Rights.

The impact forced the truck to crash outside the home of Sara Luna, 60.

The soldiers were already firing, Ms. Luna says, adding that she later counted 64 gunshots striking her home.

The gunfire lasted about 15 minutes, she says.

When it ended, she and her husband opened their front door a crack and saw soldiers standing over bleeding bodies. The soldiers ordered them inside.

Alejandro Pérez Benitez, 21, one of the two survivors, says he was in the pickup truck with his brother when the shooting started.

Another occupant, who had been shot, stumbled out of the truck outside Ms. Luna’s home, asking soldiers for an ambulance, Mr. Pérez says. They shot him again and killed him, Mr. Pérez says.

Mr. Pérez got out of the truck, at which point a soldier forced him onto his knees at gunpoint.

“‘Kill him, kill him so there is no evidence,’” he recalls another soldier yelling.

The soldiers made him lay facedown next to his brother.

Then, Mr. Pérez says, they shot his brother in the back. As he lay in a pool of his brother’s blood, Mr. Pérez could hear an ambulance — but the soldiers blocked it from arriving for over an hour.

Mr. Pérez says he put a hand on his brother’s body. It started to go cold. He kissed him.

Mr. Pérez says he was then forced to tape a confession that he had shot at the soldiers first.

Luis, 25, a barber who also survived, recalls emerging from the car with bullet wounds to his lungs and stomach. He says he was also thrown onto the pavement by the soldiers and shot in the back.

The soldiers accused him of trying to run away.

“I told them, ‘How am I going to run, I’m bleeding to death,’” he says.

Paramedics were eventually able to take Luis to a hospital, where he was put in a medically induced comma. His full name is being withheld because he fears retaliation from the military.

Humberto Suárez, the father of the American victim, woke up that morning expecting to prepare a catfish he had caught to celebrate his son starting a family.

Soon after, he received a call that his son was dead. He rushed to the scene to find his son’s bloodied remains splattered across the truck’s floor.

Days later, Mr. Suárez says, a military representative met with him and relatives of the other victims to discuss a financial settlement, a common tactic by the military, analysts say, to try to dissuade families from going to the media or to try to take cases to civilian courts.

“They did not come to say ‘we are sorry,’” he says of the meeting, which he secretly recorded and shared with The New York Times. “They came to ask how much we wanted, as if our sons were dogs.”

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