For years, Francisco Cuxum Alvarado lived a quiet life outside Boston, working as a landscaper, part of a large and well-regarded local Guatemalan community. 

But authorities say Cuxum Alvarado had a dark secret. During Guatemala’s civil war, he told investigators, he was a member of a civilian militia that allegedly raped and killed hundreds of indigenous Guatemalans. He was wanted in connection with multiple war crimes in his homeland.

Cuxum Alvarado pleaded guilty to entering the United States illegally and was sentenced last week to six months in prison. After he has served his sentence, he is expected to be extradited to Guatemala for a trial. The discovery that Cuxum Alvarado had been living openly in the middle-class town of Waltham, Mass., underscored a troubling challenge in Guatemala’s search for justice: Many of the war’s greatest alleged offenders have found impunity for decades in the United States.

“They go from mass murderers to these typical immigrant jobs,” said Jo-Marie Burt, a senior fellow at the Washington Office on Latin America who has spent years studying the aftermath of Guatemala’s 36-year civil war.

Juan Alecio Samayoa Cabrera, a resident of Providence, R.I., was turned over to Guatemalan authorities in November. He had been working as a landscaper in Providence since the 1990s. But during the civil war, he was a major paramilitary leader. Guatemala’s top human rights prosecutor called him “a very bloodthirsty person with a great level of cruelty — given the way he killed his victims.” One of his alleged victims’ daughters recognized him at a Providence Walgreens. His trial is scheduled to begin in Guatemala in January.

Gilberto Jordan, an alleged leader of the 1982 Dos Erres massacre, worked as a cook at a Florida country club. He became a U.S. citizen but was convicted in 2010 of making false statements on his naturalization forms. He, too, is expected to be deported to Guatemala and tried there for alleged war crimes.

Lawyers who represented Cuxum Alvarado, Samayoa Cabrera and Jordan did not respond to emails and calls seeking comment.

Another alleged Guatemalan war criminal worked as a martial-arts instructor in Riverside, Calif., before being detained. Another worked in a sweater factory in Santa Rosa, Calif.

Guatemalan human rights lawyers suspect that several more fugitives fleeing prosecution for war crimes are hiding in the United States.

Nancy Artola, a Guatemalan lawyer working on war-crimes cases, believes that Edgar Justino Ovalle, linked to a major massacre in the city of Coban, is hiding in the United States. Neither Ovalle nor a representative could be located for comment.

“In the 1990s to the mid-2000s, Guatemala was the land of impunity,” Burt said. “There were very few investigations into war crimes, and there was no sharing of information with the United States government about who was accused of what.”

It was during that period that many alleged Guatemalan war criminals fled to the United States. Some, such as Cuxum Alvarado and Samayoa Cabrera, applied for asylum and were denied but remained in the United States illegally afterward.

The United States backed the Guatemalan military during the civil war, which raged from 1960 to 1996, viewing it as a nearby front in the Cold War. Tens of thousands of Guatemalans were killed. When the conflict ended, Washington did little to support nascent efforts to create a war-crimes tribunal.

In 1979, the U.S. Justice Department created an office of special investigations to locate and deport former Nazis. The unit expanded to search for other alleged war criminals who had entered the United States.

But information about the Guatemalans remained limited until the 2000s, when the Central American country began holding its own trials of former military commanders. By then, the U.S. Department of Homeland Security had created its own unit to hunt down foreign fugitives in the United States, including Cuxum Alvarado.

It is unclear exactly when he first entered the United States, but he was convicted of illegal reentry in Arizona in 2004. He managed to avoid deportation and made his way to Waltham, which was known by Guatemalans across the country for its large Guatemalan population. In 2017, the American Community Survey found there were 2,349 Guatemalans in the town of 62,000. They were the largest ­foreign-born population.

“He was there and living a quiet, uneventful, unremarkable life,” Michael Ronayne, assistant special agent in charge of Homeland Security Investigations, told NBC Boston.

But while Cuxum Alvarado worked as a landscaper in Waltham, two separate cases were forming against him in Guatemala. One was brought by 36 women, who accused him and about a dozen other men of mass rape of indigenous Maya Achi women in and around the municipality of Rabinal. Another involved the killing of indigenous people in the same area.

The case of sexual assault advanced through the courts first. In court testimony, four women said they were raped by Cuxum Alvarado. Two said he participated in brutal gang rapes.

One woman interviewed by attorneys said Cuxum Alvarado took her “to a military house and raped her while threatening that if she resisted she would hang from the bloodied ropes that had been used to hang the men from Rio Negro” — referencing the massacres of indigenous Guatemalans by security personnel during the war.

The woman was pregnant at the time.

Because Cuxum Alvarado was then a fugitive, he was not tried. A judge halted the case against the other defendants, saying there was no proof that the accused were members of civilian militias. Human rights advocates in Guatemala were shocked; victims saw the ruling as a blow to their search for justice.

Cuxum Alvarado’s arrest could now reignite the case. Justice Department officials say he has admitted that he was a member of the civilian militia, known as the Rabinal Civil Defense Patrols. The militia, which often accompanied the military on its campaign to displace indigenous communities, is accused of committing some of the war’s most gruesome atrocities. “These are people who actually carried out the genocide,” Burt said.

“Cuxum Alvarado admitted that he was a member of the Rabinal PAC,” the Justice Department said in a statement last week.

“For us, this is so important,” said Ayde Claro, the lawyer in the Guatemalan women’s case. “For the women involved, this means there could finally be justice.”

Like many Guatemalan communities in the United States, Waltham is home to people from both sides of the civil war — largely indigenous victims and former militia leaders. In some places, the sides coexist uneasily. When victims see a former persecutor, they are afraid to contact authorities. Many are undocumented.

“I hate to say that there are more people like this, but there are,” said Patricia Foxen, an anthropologist who has studied the Guatemalan community in Providence. “There are still these fractures in the community. People go through their lives working at the same factories or working together as gardeners. Their kids know each other. But people still know who did what during the war.”