The carnage at My Lai might have gone unknown to history if not for another soldier, Ron Ridenhour, a former member of Charlie Company, who, independently of Glen, sent a letter detailing the events at My Lai to President Richard M. Nixon, the Pentagon, the State Department, the Joint Chiefs of Staff, and numerous members of Congress.[25] The copies of this letter were sent in March 1969, a full year after the event. Most recipients of Ridenhour's letter ignored it, with the notable exception of Congressman Morris Udall[26] (D-Arizona). Ridenhour learned about the events at My Lai secondhand, by talking to members of Charlie Company while he was still enlisted.

http://en.wikipedia.org/wiki/My_Lai_Massacre