He wrote that his witnesses claimed jeeploads of American officers "supervised the butchery." Secret CIA and Army intelligence communications reported on the Daejeon and Suwon killings as early as July 3, but said nothing about the U.S. presence or about any U.S. oversight.
In mid-August, MacArthur, in Tokyo, learned of the mass shooting of 200 to 300 people near Daegu, including women and a 12- or 13-year-old girl. A top-secret Army report from Korea, uncovered by AP research, told of the "extreme cruelty" of the South Korean military policemen. The bodies fell into a ravine, where hours later some "were still alive and moaning," wrote a U.S. military policeman who happened on the scene.
Although MacArthur had command of South Korean forces from early in the war, he took no action on this report, other than to refer it to John J. Muccio, U.S. ambassador in South Korea. Muccio later wrote that he urged South Korean officials to stage executions humanely and only after due process of law.
The AP found that during this same period, on Aug. 15, Brig. Gen. Francis W. Farrell, chief U.S. military adviser to the South Koreans, recommended the U.S. command investigate the executions. There was no sign such an inquiry was conducted. A month later, the Daejeon execution photos were sent to the Pentagon in Washington, with a U.S. colonel's report that the South Koreans had killed "thousands" of political prisoners.
The declassified record shows an equivocal U.S. attitude continuing into the fall, when Seoul was retaken and South Korean forces began shooting residents who collaborated with the northern occupiers.
When Washington's British allies protested, Dean Rusk, assistant secretary of state, told them U.S. commanders were doing "everything they can to curb such atrocities," according to a Rusk memo of Oct. 28, 1950.
But on Dec. 19, W.J. Sebald, State Department liaison to MacArthur, cabled Secretary of State Dean Acheson to say MacArthur's command viewed the killings as a South Korean "internal matter" and had "refrained from taking any action."
It was the British who took action, according to news reports at the time. On Dec. 7, in occupied North Korea, British officers saved 21 civilians lined up to be shot, by threatening to shoot the South Korean officer responsible. Later that month, British troops seized "Execution Hill," outside Seoul, to block further mass killings there.
To quiet the protests, the South Koreans barred journalists from execution sites and the State Department told diplomats to avoid commenting on atrocity reports. Earlier, the U.S. Embassy in London had denounced as "fabrication" Winnington's Daily Worker reporting on the Daejeon slaughter. The Army eventually blamed all the thousands of Daejeon deaths on the North Koreans, who in fact had carried out executions of rightists there and elsewhere.
An American historian of the Korean War, the University of Chicago's Bruce Cumings, sees a share of U.S. guilt in what happened in 1950.
"After the fact - with thousands murdered - the U.S. not only did nothing, but covered up the Daejeon massacres," he said.
Another Korean War scholar, Allan R. Millett, an emeritus Ohio State professor, is doubtful. "I'm not sure there's enough evidence to pin culpability on these guys," he said, referring to the advisers and other Americans.
The swiftness and nationwide nature of the 1950 roundups and mass killings point to orders from the top, President Rhee and his security chiefs, Korean historians say. Those officials are long dead, and Korean documentary evidence is scarce.
To piece together a fuller story, investigators of the Truth and Reconciliation Commission will sift through tens of thousands of pages of declassified U.S. documents.
The commission's mandate extends to at least 2010, and its president, historian Ahn Byung-ook, expects to turn then to Washington for help in finding the truth.
"Our plan is that when we complete our investigation of cases involving the U.S. Army, we'll make an overall recommendation, a request to the U.S. government to conduct an overall investigation," he said.
Associated Press investigative researcher Randy Herschaft in New York contributed to this report.