President Obama's visit to Argentina this week coincides with the anniversary of a dark moment in that country's history. Thursday marks 40 years since a 1976 military coup that ushered in that country's so-called Dirty War, when as many as 30,000 Argentines were killed or disappeared during a seven-year dictatorship.

Human rights groups want the U.S. to divulge what it knew back then. The president is now promising that he will declassify new documents.

"I'm launching a new effort to open up additional documents from that dark period," Obama told a joint news conference in Buenos Aires on Wednesday with Argentina's president, Mauricio Macri. "We previously declassified thousands of records from that era, but for the first time now, we will declassify military and intelligence records as well."

That's welcome news to Carlos Osorio. At his office in the George Washington University library, he shows off boxes and boxes of government documents. These are papers that his research organization, the National Security Archive, forced the U.S. government to declassify more than a decade ago.

Some appear to show former Secretary of State Henry Kissinger giving Argentina's coup leaders the green light to go after their enemies.

"In early 1976 through January 1977, Henry Kissinger took U.S. policy into his hands," Osorio says. "And he was deliberately telling the [Argentine] military at every point through the year, 'We will support you.' "

Although Kissinger has denied any complicity, Osorio says the State Department quietly released U.S. security assistance to Argentina — even as the U.S. ambassador at the time was reporting hundreds of human rights abuses.

"So they start to disappear these people — scientists, nuclear scientists, painters, journalists, famous historians start to disappear, and people ask the embassy to help with this," Osorio says.

But while the ambassador was raising serious human rights concerns, Kissinger was sending a different message to Argentina's foreign minister, Osorio says: "Whatever needs to be done, do it quickly."

He says this is just a glimpse of U.S. policy from that era. The military and intelligence documents that President Obama promised to release are expected to be even more revealing.

Argentines are eager for any scraps of information about family members who disappeared in the Dirty War years, says Elisa Massimino, the president and CEO of Human Rights First.

"They are looking for answers," she says. "They are also, I think, looking for a deeper and more accurate understanding of the role of the United States."

Massimino says some activists in Argentina were angry when they first learned that Obama would visit on the anniversary of the military coup. But Human Rights First saw the visit as an opportunity to press the White House.

"The release of these documents is hugely important," she says, "not only for the people of Argentina, who are continuing to struggle to come to grips with that period — but also it's important for Americans, too, because we have to come to terms with our own role in the Dirty War."

Osorio, who says he will advise the White House on what to look for and where, believes the administration could come away from this awkwardly timed trip looking good.

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