President Richard Nixon may have been personally behind an attempt to cover up the brutal killing of over 500 Vietnamese civilians by American soldiers in the South Vietnam village of My Lai in 1968, according to historians who spoke with CBS journalist Evie Salomon.

Handwritten notes by Nixon's chief of staff H.R. "Bob" Haldeman taken during a December 1, 1969 meeting with the president read: "Task force - My Lai," adding beneath "dirty tricks [...] not too high a level" and "discredit one witness," in order to "keep working on the problem."

The note "reads like a threatening to-do list," writes Salomon.

Ken Hughes, a researcher with the University of Virginia's Miller Center Presidential Recording Program, told Salomon that "Haldeman's note is an important piece of evidence that Nixon interfered with a war-crime prosecution."

Marking the 46th anniversary of the March 16 My Lai massacre, Salomon spoke with a number of historians who conclude that these documents are evidence that Nixon attempted to sabotage the court-martial trials by burying the testimony of American helicopter pilot Hugh Thompson—who witnessed the massacre and attempted to report on the slaughter of hundreds of unarmed men, women and children.

Salomon reports:

Thompson testified about the massacre in the U.S. government's court-martial trials, but according to author Trent Angers, two Congressmen who were working in concert with Nixon, managed to seal that testimony in order to damage the cases against the culprits of My Lai. Whether it was one of the "dirty tricks" Nixon prescribed in Haldeman's 1969 meeting note is a matter of debate for historians.

James Rife, a senior historian at History Associates Inc., helped author Trent Angers find the Haldeman meeting notes, which are described in detail in "The Forgotten Hero of My Lai."

"I would not characterize [the "dirty tricks" note] as a smoking gun, but it's pretty strong," Rife says. "I don't think we'll ever find an actual document that can make the absolute final link between Nixon and Hugh Thompson."

According to historian Ken Hughes, it's the historical context that makes for a convincing argument. He calls My Lai "a political threat to Nixon," and points out that a substantial part of Nixon's support base refused to believe that killing civilians in a war zone was a crime. According to Hughes, Nixon's approval rating dropped by 10 points after Lieutenant William Calley received a life sentence for murdering civilians at My Lai.

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