When did the U.S. military begin official investigations into the My Lai Massacre?

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A group of frightened Vietnamese women and children stand close together outdoors during the My Lai Massacre in the Vietnam War. — My Lai Massacre Vietnamese citizens photographed during the My Lai Massacre, March 16, 1968.

Official investigations into the My Lai Massacre were sparked by a report mailed on March 29, 1969, by Ronald Ridenhour, a former helicopter door gunner who had gathered eyewitness testimony about the event.1

While a cursory investigation had been conducted immediately following the massacre in March 1968, it was dismissed by Col. Oran Henderson, who characterized the operation as a success and falsely reported that civilian deaths were accidental. Following Ridenhour's 1969 report to Congress and the Pentagon, the U.S. military launched formal investigations into both the massacre and the subsequent cover-up. Additionally, a parallel inquiry into the cover-up at various levels of command was launched by Lieut. Gen. William Peers on November 26, 1969.1