On November 25, 1960, three sisters—Patria, Minerva and María Teresa Mirabal—were reported killed in an “automobile accident.” Reports said a car they were riding in plunged over a cliff in the Dominican Republic.

At least, that was the story in El Caribe, a newspaper sanctioned by the government of Rafael Trujillo, the brutal dictator who had seized control of the island nation in a military coup 30 years earlier. In reality, the Mirabal sisters were active members of the growing underground resistance against Trujillo’s regime, and everyone knew their deaths were no accident.

As middle-class women, wives and mothers, the Mirabal sisters didn’t seem like obvious revolutionaries. Patria, Minerva and María Teresa, along with their sister Dedé, grew up in the town of Ojo de Agua, Salcedo Province, where their parents owned and operated a successful farm, along with a coffee mill and general store.

After attending the Colegio Inmaculada Concepción, a Catholic boarding school in the city of La Vega, Minerva headed to college in Santo Domingo, the capital, to study law. By that time, she had become increasingly aware of the injustices that existed in the Dominican Republic during the Trujillo era.

Known as “El Jefe” (“the Boss”) or “el Chivo” (“the Goat”), Trujillo was the commander in chief of the army before he seized power in 1930. The prosperity, modernization and stability his regime brought to the country came at a high price: Trujillo took over the country’s economy, including production of such goods as salt, meat, tobacco and rice, and channeled the profits to his own family and supporters. Civil and political liberties disappeared, and only one political party, the Dominican Party, was allowed to exist.

Trujillo’s fearsome secret police rooted out dissenters, using tactics of intimidation, imprisonment, torture, kidnapping and rape of women, and murder. His regime would ultimately be responsible for tens of thousands of deaths, including the massacre of an estimated 20,000 Haitians living near the border between Haiti and the Dominican Republic in 1937.

“There was a huge danger during that period,” says Elizabeth Manley, an associate professor of history at Xavier University of Louisiana and the author of The Paradox of Paternalism: Women and Authoritarian Politics in the Dominican Republic (2017). “People were being disappeared and jailed and killed.”

Resistance was still building to the regime, both among exiled groups of Dominicans abroad, and at home. The majority of those involved were men, but many women also joined the movement, including the Mirabal sisters. By the end of 1949, Minerva had been arrested for suspected opposition activities; she also reportedly angered Trujillo by rejecting his sexual advances. At the University of Santo Domingo, she met fellow activist Manolo Tavárez Justo, and they married in 1955.

Minerva and her husband became resistance leaders, and Patria, María Teresa and their husbands soon joined them. In early 1960, they helped form the 14th of June Movement, named for the date of a failed insurrection against Trujillo led by a group of exiled Dominicans with the support of the Cuban government the previous year. Shortly after the movement was officially organized, Trujillo began mass arrests of resistance figures, including the sisters and their husbands, though he later freed female prisoners as a supposed gesture of his leniency.

After the attempt to assassinate Venezuela’s President Romulo Betancourt on Trujillo’s orders that June, the Organization of American States (OAS) cut diplomatic ties with and imposed sanctions on the Dominican Republic, and the United States withdrew its support of the regime. Trujillo was also losing ground at home, with the powerful church condemning his government’s actions.

Against this backdrop, the Mirabal sisters traveled on November 25, 1960, to visit their husbands in prison in Puerto Plata. On the way back, Trujillo’s henchmen stopped their car along a mountain road and killed their driver, Rufino de la Cruz, before kidnapping the sisters at gunpoint, beating and strangling them. The assassins then put the four bodies back in the car and pushed it over a cliff to make it look like an accident.

The Butterflies, as the Mirabals were known, instantly became martyrs to the revolutionary cause, helping solidify resistance to Trujillo both at home and abroad.

“Killing women...was just beyond what people could stomach, and that catalyzed a lot of people to become more active in the movement,” says Manley. Trujillo had painted himself as a champion of women and of mothers, granting full female suffrage in 1942 and sending one of the first female delegates (from any country) to the United Nations in 1945. “He touted these things and said they were an element of his progressivism,” says Manley. “So this failure to protect women and [going] against this kind of maternal politics was a serious blow.”

On May 30, 1961, seven assassins (including former members of the armed forces) ambushed the dictator’s car along a coastal highway and killed him. Though Trujillo’s death did not immediately bring democracy to the Dominican Republic—his successor, Joaquin Balaguer, continued the authoritarian tradition until the late 1970s—the country did not return to the level of brutal repression experienced during his reign.

Dedé Mirabal, who had largely maintained her distance from the resistance, survived the Trujillo regime and went on to raise her sisters’ children, along with her own. Minerva’s daughter, Minou Tavárez Mirabal, became a congressional representative and vice foreign minister, while Dedé’s son Jaime David Fernández Mirabal served as vice president of the Dominican Republic (1996-2000).

The fame of the Mirabal sisters, fueled by Julia Alvarez’s historical novel In the Time of the Butterflies (1994), spread worldwide. In 1999, the United Nations designated November 25, the anniversary of their deaths, as International Day for the Elimination of Violence Against Women. Dedé Mirabal also ensured her sisters’ legacy, managing a museum out of their childhood home, the Casa Museo Hermanas Mirabal. She died in 2014, at the age of 88.