The American colonel, troubled by what he was hearing, tried to stall at first. But the declassified record shows he finally told his South Korean counterpart it "would be permitted" to machine-gun 3,500 political prisoners, to keep them from joining approaching enemy forces.

In the early days of the Korean War, other American officers observed, photographed and confidentially reported on such wholesale executions by their South Korean ally, a secretive slaughter believed to have killed 100,000 or more leftists and supposed sympathizers, usually without charge or trial, in a few weeks in mid-1950.

Extensive archival research by The Associated Press has found no indication Far East commander Gen. Douglas MacArthur took action to stem the summary mass killing, knowledge of which reached top levels of the Pentagon and State Department in Washington, where it was classified "secret" and filed away.

Now, a half-century later, the South Korean government's Truth and Reconciliation Commission is investigating what happened in that summer of terror, a political bloodbath largely hidden from history, unlike the communist invaders' executions of southern rightists, which were widely publicized and denounced at the time.