South Korean forces, sometimes observed by US Army officers, killed the prisoners because they were suspected of Communist sympathies.
Information about the South Korean Government's mass executions was suppressed for decades under the country's former military rulers.
The documents - two brief US Army reports on shootings in the village of Dokchon and high-level correspondence that resulted from them - have provided vital evidence of the killings.
Brutal mass killings
The documents, obtained by the Associated Press, bear out testimony from witnesses.
Victims' families have petitioned the South Korean Government for investigations of at least 10 alleged civilian massacres.
"Our first goal is simple, to let the world know that this
massacre of a gigantic scale really took place," said Lee
Bok-ryong, 71, who lost his father.
Villagers in the Dokchon area remember truckloads of civilians, trussed together, brought to the hills and executed by South Korean military police.
One of the documents showed that the US ambassador to South Korea at the time, John Muccio, urged President Syngman Rhee and Defense Minister Shin Sung-mo to end summary executions, deemed illegal and inhumane.
South Korean soldiers had shown "extreme cruelty" toward the prisoners at Dokchon, a US military police investigator, Sergeant Frank Pearce, said in a written report to his company commander on the shootings.
He and other American witnesses reported that 200 to 300 prisoners, including women and a girl 12 or 13 years old, were killed by South Korean military police on 10 August 1950 on a mountain near the town.
A South Korean officer told the Americans the prisoners were "spies" - not North Korean soldiers or guerrillas.
Sgt Pearce, who went to the scene after hearing gunfire, said the South Korean soldiers placed 20 prisoners at a time on the edge of a cliff and shot them in the back of the head.
Because some of the soldiers had poor aim, not all prisoners died immediately.
"About three hours after the executions were completed, some of the condemned persons were still alive and moaning. The cries could be heard coming from somewhere in the mass of bodies piled in the canyon," Sgt Pearce wrote in his report.
'No time for trials'
Retired South Korean Admiral Nam Sang-hui, now living in New York, told the Associated Press that in early July 1950 he authorised three ships to carry 200 people out to sea off the eastern port of Pohang.
There they were shot by police and their bodies were thrown into the sea, weighted with stones.
"There was no time for trials for them. Communists were streaming down. It [summary execution] was a common practice at that time," he said.
"It happened during a critical situation for South Korea. We should not judge these incidents through the standards of peacetime," the retired admiral added.