Tensions along the border ran high in the final years of the Mexican Revolution. Violence spilled across from northern Mexico into the Texas frontier, where recent Anglo settlers clashed with Mexican Americans who had lived there for generations. The complex dynamics between newcomers and longtime residents often led to retaliatory attacks—then everything came to a head.

On the morning of January 28, 1918, a band of Texas Rangers, U.S. cavalry soldiers, and a group of local ranchers entered the small farming community of Porvenir—a town in Presidio County with a population of roughly 140. They forced several residents from their homes before leading fifteen unarmed men and boys to a nearby hill where they were executed.

The massacre went unreported for weeks, and didn’t come to light until Captain J.M. Fox, of the Texas Rangers, told command that they’d been ambushed by locals suspected of having ties to a raid at a nearby ranch a month earlier; he characterized them as “thieves, informers, spies, and murderers.”

For nearly a century, Fox’s account was widely accepted as fact. But in recent years, researchers and descendants of the massacre began unraveling the truth of what transpired that day. In the documentary Porvenir, Texas, streaming on PBS until Oct. 19, the late director Andrew Shapter examines the dynamics that led up to the tragedy, along with the scars it left behind, and reconsiders something that’s long been deemed historical truth.

In its 55-minute run time, the documentary sets up the conflicts that led to the massacre, before portraying a powerful reenactment of what happened in the early hours of January 28. Through interviews with archaeologists, historians, and descendants of the deceased, the film follows the many attempts to seek justice in the years following the attack. It then pivots to today, where the stories of the descendants and their fight to have their history acknowledged serves as the documentary’s final act.

What happened in Porvenir was far from the first act of violence the Texas Rangers perpetrated against Mexican Americans—but by the time of this attack, the community was desperate for justice. Crucially, the documentary dives into archaeological evidence uncovered in 2015 by Sul Ross State University archaeologist David Keller that proved that every bullet fired during the attack was government-issued—a fact that essentially absolved the people of Porvenir of what had allegedly provoked the attack.

Another critical piece of evidence is the reenacted account of Harry Warren, an Anglo schoolmaster in Porvenir whose father-in-law was killed in the massacre. Enraged by the senseless act of violence, Warren sought justice by writing detailed notes about those who were murdered, complete with testimony from the surviving members of their families.

Years later, Warren’s account served as confirmation of what transpired that day. One Porvenir descendant, Arlinda Valencia, lost her great-grandfather Longino Flores in the attack; her great uncle, Juan Flores, just twelve years old at the time, was the sole eyewitness. At a family funeral, she said family members whispered about Longino having been murdered at the hands of the Texas Rangers, but it seemed too outlandish to be true.

When she began looking into it as she got older, she found her great-grandfather’s name on the list of those killed, as well as her great-grandmother’s account as told to Warren. But she also uncovered information that blamed her ancestors for the attack.

“I was shocked,” Valencia says. “They weren’t bandits, they weren’t squatters, it’s just not true. My great-grandfather was murdered and, later, my great-grandmother killed herself. That traumatized my father. My whole family suffered. And for years, no one knew why.”

In one of the film’s most powerful scenes, an archival clip shows Juan Flores describing how his father’s face was unrecognizable after the massacre. Flores went nearly his entire life without talking about what he saw that morning, suffering recurring nightmares and living in fear of a similar attack. He knew they were innocent. “What could we do?” he asks. “Who could we tell?”

The documentary also shows the attempts to silence people who questioned or spoke out about the massacre. It follows the efforts of Representative José Tomás Canales, of Brownsville, to hold the Rangers accountable in 1919. Despite being openly threatened and stalked by legendary Texas Ranger Frank Hamer (known for leading the hunt of Bonnie Parker and Clyde Barrow), Canales filed nineteen charges of misconduct against the organization, launching a full-blown investigation in the Texas Legislature. Within two weeks, the representative had called 90 people to testify about the Rangers’ brutal, bloody tactics along the border.

Pulling from transcripts that had been sealed for decades, the film puts these chilling testimonies on full display. In one of them, a witness described seeing a man hanging from a tree, “his body riddled with bullets.” Another saw “bullets passing through the heads of many [Mexicans].”

Ultimately, the Rangers were found guilty of gross violations of civil and criminal laws, including the killing of up to 5,000 people, largely of Mexican descent, between 1914 and 1919. Led by Canales, the investigation helped pave the way for stricter recruitment guidelines that transformed the Rangers into the elite organization it later came to be known as. Yet none of the Rangers implicated in the Porvenir massacre were ever found guilty of murder.

This particular injustice serves as the backdrop for the film’s closing chapter. In 2018, descendants from across the country gathered to meet at the Texas Capitol in honor of the massacre’s one hundredth anniversary. Many of them expressed pain and frustration that their ancestors had never formally been vindicated.

Since learning the truth about her family’s involvement in the massacre, Valencia, a teacher of 32 years, has dedicated her time to running a website with lesson plans on Porvenir for interested teachers, as well as information about the victims to connect potential descendants to their own history. She’s still holding out hope that the Rangers will one day publicly acknowledge their role in the attack, too.

“Those fifteen men who were killed were the straw that broke the camel’s back,” Valencia says. “Their deaths made the Rangers into the group that we know today, but they need to acknowledge what they did and apologize, so that my great-grandfather did not die in vain.”

Christina Fernandez Shapter, who produced the film alongside her husband Andrew, says that while they started on the documentary three years ago, the themes of injustice and racial discrimination that emerge through this story have since become even more relevant. “There’s so much rhetoric and misinformation these days about the presence of Latinos in this country,” Shapter says. “But what this film makes clear is that we have a long history of people who never crossed a border—it crossed them.”