Artist and activist Joel Filártiga, right, reads a poem in honor of his son at the Paraguayan Senate in 2018. (Alejandro R. Otero/EFE/Alamy Live News)

The authorities said it was a crime of passion.

In Paraguay, where the military government of Gen. Alfredo Stroessner had cracked down on political dissidents while turning the country into a haven for war criminals and smugglers, a police officer was alleged to have killed a 17-year-old upon finding him with the officer’s wife.

But the young man’s body was bruised, battered and burned from electric shocks — signs of torture that were unmistakable to his father, Joel Filártiga. A physician, painter and human rights advocate, Dr. Filártiga had spent much of his career opposing the Stroessner regime, treating indigent and indigenous patients while speaking out against government abuses and facing imprisonment and torture himself.

If the 1976 killing was meant to silence him, it backfired spectacularly, reinvigorating Dr. Filártiga’s decades-long quest for justice in Paraguay. His efforts culminated with a watershed human rights case in the United States, where his family was awarded $10 million in a wrongful-death suit that hinged on an obscure 18th-century statute — and that launched numerous lawsuits targeting torture, genocide and other abuses around the world.

Dr. Filártiga, whose work inspired the 1991 HBO film “One Man’s War,” starring Anthony Hopkins, was 86 when he died July 5 at a hospital in Asuncion, the capital of Paraguay. Local media reported the death, and information on the cause was not immediately available.

The gray-bearded Dr. Filártiga was among the most prominent critics of Stroessner, who ruled the landlocked South American country for 35 years before being ousted in a 1989 coup. A truth commission established by Paraguay’s Congress later catalogued more than 128,000 victims of the regime, including 18,772 cases of torture and 336 forced disappearances.

The son of a wealthy tobacco exporter, Joel Holden Filártiga Ferreira was an unlikely dissident. He was born in the town of Ybytymi, 60 miles southeast of Asuncion, on Aug. 15, 1932, and studied at the National University of Asuncion and the Albert Einstein College of Medicine in New York.

In line to inherit a life of privilege, including two private planes and a sprawling country estate, he instead decamped to the southeastern community of Ybycui and devoted himself to the poor, serving as the sole physician for 40,000 people. He was often paid “with eggs, chickens, beans and potatoes,” according to a 1982 account in The Washington Post, and encouraged his patients to vote, although elections were scarcely free and open.

In part, he financed his work by selling his paintings to buyers in the United States, an ally of Stroessner’s anti-communist regime. On trips abroad, he also lectured on government abuses, further angering state officials. He was arrested three times for his political opposition, according to court filings, and tortured at least once in the years before the March 29, 1976, killing of his son, who shared the same name and was known as Joelito.

By the family’s account, Joelito was taken in the middle of the night from his home in Asuncion by Americo Peña-Irala, the inspector general of police, and led either directly to Peña-Irala’s house, two doors down, or first ushered to a police station, where Dr. Filártiga suspected he may have been killed by accident when the torture became too severe.

“Joelito was given electric shocks on his genitals, his nails were ripped, his wrists were destroyed, his body was completely bruised and even a wire was placed on his penis,” Dr. Filártiga’s daughter Dolly, who was staying with her brother at the time, later told the radio station Monumental.

About 3 a.m., Dolly Filártiga said, she was roused by police and taken to Peña-Irala’s home. In the back of the house was her brother’s body. As she left the building, screaming, she was approached by Peña-Irala, who allegedly told her: “Here you have what you have been looking for so long and what you deserve. Now shut up.”

Dr. Filártiga enlisted doctors to conduct an independent autopsy, took photos of the body and invited friends to a viewing, and distributed pictures and details of the killing to newspapers. His wife and daughter were briefly imprisoned, sentenced to jail without trial. And after Dr. Filártiga filed a lawsuit against the police, his lawyer was arrested, shackled at police headquarters and confronted by Peña-Irala, who allegedly threatened to kill him and Dr. Filártiga if they continued with the lawsuit.

Soon after, Peña-Irala vanished, apparently fleeing to the United States. He was eventually tracked down by Dolly Filártiga, who moved to Washington and learned that her brother’s alleged killer was living in Brooklyn, where he had overstayed his visa. In April 1979, based on a tip from Dolly Filártiga, he was arrested and received a deportation order.

The next day, Dr. Filártiga and his daughter filed a wrongful-death suit in U.S. District Court in Brooklyn. Although both parties were Paraguayan and the death occurred overseas, Dr. Filártiga’s legal team — led by Peter Weiss and Rhonda Copelon of the Center for Constitutional Rights in New York — argued that Joelito’s torture and killing violated international law and was not simply a matter for Paraguayan courts.

Citing the Alien Tort Statute, adopted as part of the Judiciary Act of 1789, they effectively likened torturers to 18th-century pirates and slave traders, who were deemed “enemies of all mankind” and liable to prosecution wherever they were found.

“A lot of colleagues thought we were slightly insane,” Weiss recalled in a phone interview. “There really had never been a case with these facts.” Indeed, the district judge granted Peña-Irala’s motion to dismiss the complaint for lack of jurisdiction, enabling him to return to Paraguay.

But the decision was reversed in 1980 by the U.S. Court of Appeals for the 2nd Circuit, with help from an amicus brief filed by the State Department, which argued that the case should proceed.

In an opinion written by Irving Kaufman, the same judge who presided over the espionage trial of Julius and Ethel Rosenberg, the court declared, “Our holding today, giving effect to a jurisdictional provision enacted by our First Congress, is a small but important step in the fulfillment of the ageless dream to free all people from brutal violence.”

Dr. Filártiga and his daughter were ultimately granted a default judgment and awarded $10 million in damages. By most accounts, they were never able to collect.

Nonetheless, the case “energized human rights lawyers not only in the United States but throughout the world,” Weiss said. “In the next several years, courts in Europe and Latin America used the Filártiga case as a precedent for making the case that some violations of human rights principles are so egregious that they should be subject to prosecution and conviction regardless of where the acts occurred.”

In a 2017 interview with the podcast “More Perfect,” former State Department legal adviser John Bellinger described the statute as “the source of almost all significant human rights litigation in the United States and indeed in the world” over the past 35 years.

In the United States, the Alien Tort Statute served as the basis of lawsuits against government officials in countries such as Argentina, Chile, El Salvador, the Philippines and the former Yugoslavia, and eventually against corporations such as Royal Dutch Petroleum. Its effectiveness as a human rights tool was severely weakened in 2013, when the U.S. Supreme Court ruled that the statute generally does not apply to conduct overseas.

Dr. Filártiga continued his work in recent years, with a focus on pesticides that pose a risk to farmworkers. In November, he was honored by the Paraguayan Senate for “his unwavering fight for health, freedom and justice.”

A complete list of survivors was not immediately available, although according to Newsday, he had three daughters named for characters in the Leo Tolstoy novel “Anna Karenina.” He told the newspaper in 2004 that he thought of his lost son “all the time” and, while in Asuncion, had recently spotted Peña-Irala. The former neighbor did not seem to recognize him.

“I hear he doesn’t sleep,” Dr. Filártiga said.