South Korean political prisoners are about to be executed by South Korean soldiers and police at Daejeon, South Korea, in July 1950. The Truth and Reconciliation Commission of Korea is investigating the estimated mass killing of at least 100,000 political prisoners and peasants in the country from 1950 to 1951.

DAEJEON, South Korea — Grave by mass grave, South Korea is unearthing the skeletons and buried truths of a cold-blooded slaughter from early in the Korean War, when this nation's U.S.-backed regime killed untold thousands of leftists and hapless peasants in a summer of terror in 1950.

With U.S. military officers sometimes present, and as North Korean invaders pushed down the peninsula, the southern army and police emptied South Korean prisons, lined up detainees and shot them in the head, dumping the bodies into hastily dug trenches. Others were thrown into abandoned mines or into the sea. Women and children were among those killed. Many victims never faced charges or trial.

The mass executions — intended to keep possible southern leftists from reinforcing the northerners — were carried out over mere weeks and were largely hidden from history for a half-century. They were "the most tragic and brutal chapter of the Korean War," said historian Kim Dong-choon, a member of a 2-year-old government commission investigating the killings.

Hundreds of sets of remains have been uncovered so far, but researchers said they are only a tiny fraction of the deaths. The commission estimates at least 100,000 people were executed in a South Korean population of 20 million.

That estimate is based on projections from local surveys and is "very conservative," Kim said. The true toll might be twice that or more, he said.

In addition, thousands of South Koreans who allegedly collaborated with the communist occupation were slain by southern forces later in 1950, and the invaders staged their own executions of rightists.

Through the postwar decades of South Korean right-wing dictatorships, victims' fearful families kept silent about that blood-soaked summer. American military reports of the South Korean slaughter were stamped "secret" and filed away in Washington. Communist accounts were dismissed as lies.

Only since the 1990s, and South Korea's democratization, has the truth begun to seep out.

In 2002, a typhoon's fury uncovered one mass grave. Another was found by a television news team that broke into a sealed mine. Further corroboration comes from a trickle of declassified U.S. military documents, including U.S. Army photographs of a mass killing.