It was the worst massacre in the modern history of Latin America: As many as a thousand Salvadoran civilians, many of them younger than 6, killed by a right-wing military dictatorship backed by the United States.

Thirty-nine years later, the suspected killers are on trial, and the judge is seeking a crucial piece of evidence: U.S. government records related to the 1981 El Mozote massacre.

The prosecution has been lauded by both the U.S. government and the relatives of victims. Now the Trump administration will have to decide what to hand over. So far, it has offered nothing.

The Mozote trial, which began in 2016, is considered a milestone in El Salvador’s reckoning with its dark history. The massacre remains the most infamous killing in the country’s bloody 12-year civil war, its survivors silenced by a succession of governments, its victims left to decompose in a mass grave.

The process has begun to reveal what happened in the northeastern village of El Mozote. Evidence presented by prosecutors contradicts the original Salvadoran and American accounts of the massacre, implicating a U.S.-trained special forces battalion.

Jean Manes, a former U.S. ambassador to El Salvador, has called the trial “an important, positive step for rule of law and ending impunity” in the country.

But as Judge Jorge Guzmán Urquilla sorted through reams of evidence and testimony, he found that a key piece was missing: U.S. documents that might shed light on how the massacre was planned and executed. In January, he sent a letter to Secretary of State Mike Pompeo.

“As a judge,” Guzmán wrote, “I would hope that it would provide me with greater certainty and clarity on these heinous acts that are now part of our country’s history, something we are not proud of, but which the historical record will demand we adjudicate.”

At a hearing Wednesday, Guzmán spoke again about the need for the United States to open its archives.

“This information could be very valuable to us,” he said. “It could clarify what happened.”

The State Department declined to comment.

“We do not comment on the secretary’s correspondence,” a State Department spokesman said.

As Cold War attention shifted in the early 1980s to Central America, the Reagan administration bankrolled the Salvadoran government’s war against leftist guerrillas, making what was then a nation of fewer than 5 million one of the biggest beneficiaries of U.S. aid. The U.S. military trained Salvadoran officers at the School of the Americas at Fort Benning, Ga., and sent U.S. military advisers to El Salvador. In the guerrillas, President Ronald Reagan saw the specter of communism on the doorstep of the United States.

By early 1981, Salvadoran troops were based near El Mozote and patrolling near a guerrilla camp. That December, the U.S.-trained Atlacatl Battalion was dispatched to the area to fight guerrillas. But what happened in the village on Dec. 11 was much different.

“Several hundred civilians, including women and children, were taken from their homes in and around this village and killed by Salvadoran Army troops,” The Washington Post reported in January 1982. Survivors said troops herded villagers into two groups — men in one, women and children in the other — took them away and shot them.

María del Rosario López Sánchez, now 73, remembers a group of 60 soldiers entering the village and targeting residents. She lost 24 family members in a single day. She barely escaped on foot.

“I saw the soldiers,” López Sánchez told The Post. “They had forced all the people outside their houses, they had them under a mango tree. I saw my family, I saw what happened. They shot them.”

At the time, the United States refused to implicate its partners.

“Civilians did die during Operation Rescate, but no evidence could be found to confirm that government forces systematically massacred civilians in the operation zone,” Thomas Enders, the assistant secretary of state for inter-American affairs at the time of the attack, told Congress in February 1982.

It became clear that the Salvadoran military was responsible for the massacre. But details were vague. The commanders of the Atlacatl Battalion remain free. So do the former senior defense officials who allegedly issued orders to the battalion. In the 1990s, the country approved an amnesty that protected war criminals. That law was declared unconstitutional in 2016, clearing the way for the trial.

The defendants include 16 former commanders, including a former minister of defense. Last year, Guzmán added charges of torture, forced displacement and forced disappearance. But how involved were senior Salvadoran officials? Was the massacre an improvised shock of violence, as some military officers have suggested, or was it organized in advance by high-level commanders and politicians?

“The U.S. documents would clarify the role of the Salvadoran chain of command,” said David Morales, a lawyer representing victims’ relatives. “They could reveal why this population was selected for extermination and how these massive crimes occurred.”

The United States has long acknowledged the role of the Salvadoran military in abuses in the war that ran from 1979 to 1992. This year, the State Department sanctioned 13 former soldiers for the killing of six Jesuit priests and two others in 1989.

The Clinton administration declassified hundreds of documents related to the U.S. role in the Salvadoran civil war. Researchers have obtained other documents through Freedom of Information Act requests. Many have been catalogued as evidence in the trial. But analysts think the U.S. government is withholding key documents.

“There is no doubt that we would find incredibly strong and relevant information related to this case if the U.S. opened up its archives,” said Kate Doyle, a senior analyst at the National Security Archive in Washington.

“The CIA and defense attaches had a direct line to senior Salvadoran military structures, with direct access to army commanders on the field,” Doyle said. “They had detailed information about what the regime was doing in its counterinsurgency campaign.”

The CIA declined to comment.

Members of Congress have been pushing the Trump administration to respond to Guzmán’s request. Even before Guzmán’s letter was sent, the House Appropriations Committee directed Pompeo last year to “assist the judicial authorities of El Salvador in the investigation and prosecution of those responsible for the El Mozote massacre.”

 The committee specified the “identification and provision of related documents, correspondence, reproductions of Salvadoran documents, and other similar materials from January 1981 to January 1983.”

Declassified U.S. documents have been used in war crimes trials throughout Latin America in recent decades. Last year, President Trump personally handed over to then-President Mauricio Macri a trove of U.S. documents related to human rights abuses under Argentina’s military dictatorship between 1976 and 1983.

George Mason University political scientist Jo-Marie Burt, a senior fellow at the Washington Office on Latin America, urged the same action for the Mozote trial.

“These outward demonstrations of [U.S.] support are meaningless unless they are accompanied by real action,” she said. “And declassifying internal government documents would be one very clear action.”

Guzmán and the prosecutors have also struggled to obtain internal documents from the Salvadoran government.

“They claim it was a spontaneous operation, an undocumented one,” Guzmán told The Post.

In the absence of Salvadoran documents, he said, the U.S. government material is even more important.

“That information could strengthen the case against the accused,” he said. “It would make it clear not only that a massacre occurred, but who was behind it, who planned it.

“The victims are getting older. Some of them are dying. They deserve answers.”

Sieff reported from Mexico City.