My understanding is that Powell's investigation was of a complaint from a GI, Tom Glen, alleging that he had witnessed many incidents of cruelty and abusive treatment of civilians in the course of his tour of duty. Glen was not at My Lai himself and did not refer to My Lai in his letter of complaint. However his letter was written not long after the My Lai incident and rumours about it were circulating among the troops.
If Powell had performed a legitimate investigation and follow up on Glen's allegations instead of performing a whitewash to cover his superiors, many allege that it would have been hard for Powell to have not uncovered the story about My Lai
In 1963, Capt. Colin Powell was one of those advisers, serving a first tour with a South Vietnamese army unit. Powell's detachment sought to discourage support for the Viet Cong by torching villages throughout the A Shau Valley. While other U.S. advisers protested this countrywide strategy as brutal and counter-productive, Powell defended the "drain-the-sea" approach then -- and continued that defense in his 1995 memoirs, My American Journey. (See The Consortium, July 8)
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But a test soon confronted Maj. Powell. A letter had been written by a young specialist fourth class named Tom Glen, who had served in an Americal mortar platoon and was nearing the end of his Army tour. In a letter to Gen. Creighton Abrams, the commander of all U.S. forces in Vietnam, Glen accused the Americal division of routine brutality against civilians. Glen's letter was forwarded to the Americal headquarters at Chu Lai where it landed on Maj. Powell's desk.
Glen's letter contended that many Vietnamese were fleeing from Americans who "for mere pleasure, fire indiscriminately into Vietnamese homes and without provocation or justification shoot at the people themselves." Gratuitous cruelty was also being inflicted on Viet Cong suspects, Glen reported.
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Glen's letter echoed some of the complaints voiced by early advisers, such as Col. John Paul Vann, who protested the self-defeating strategy of treating Vietnamese civilians as the enemy. In 1995, when we questioned Glen about his letter, he said he had heard second-hand about the My Lai massacre, though he did not mention it specifically. The massacre was just one part of the abusive pattern that had become routine in the division, he said. Above quotes from:
http://www.consortiumnews.com/archive/colin3.html