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 say 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," said Kim. The true toll may 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 outside this central South Korean city.

Now Kim's Truth and Reconciliation Commission has added government authority to the work of scattered researchers, family members and journalists trying to peel away the long-running cover-up. The commissioners have the help of a handful of remorseful old men.

"Even now, I feel guilty that I pulled the trigger," said Lee Joon-young, 83, one of the executioners in a secluded valley near Daejeon in early July 1950.

The retired prison guard told the AP he knew that many of those shot and buried en masse were ordinary convicts or illiterate peasants wrongly ensnared in roundups of supposed communist sympathizers. They didn't deserve to die, he said. They "knew nothing about communism."

In January, then-President Roh Moo-hyun, under whose liberal leadership the commission was established, formally apologized for the more than 870 deaths confirmed at Ulsan, calling them "illegal acts the then-state authority committed."

The commission, with no power to compel testimony or prosecute, faces daunting tasks both in verifying events and identifying victims, and in tracing a chain of responsibility. Under Roh's conservative successor, Lee Myung-bak, whose party is seen as democratic heir to the old autocratic right wing, the commission may find less budgetary and political support.

The roots of the 1950 bloodbath lie in the U.S.-Soviet division of Japan's former Korea colony in 1945, which precipitated north-south turmoil and eventual war.

In the late 1940s, President Syngman Rhee's U.S.-installed rightist regime crushed leftist political activity in South Korea, including a guerrilla uprising inspired by the communists ruling the north. By 1950, southern jails were packed with up to 30,000 political prisoners.

The southern government, meanwhile, also created the National Guidance League, a "re-education" organization for recanting leftists and others suspected of communist leanings. Historians say officials met membership quotas by pressuring peasants into signing up with promises of rice rations or other benefits. By 1950, more than 300,000 people were on the league's rolls, organizers said.

North Korean invaders seized Seoul, the southern capital, in late June 1950 and freed thousands of prisoners, who rallied to the northern cause. Southern authorities, in full retreat with their U.S. military advisers, ordered National Guidance League members in areas they controlled to report to the police, who detained them. Soon after, commission researchers say, the organized mass executions of people regarded as potential collaborators began - "bad security risks," as a police official described the detainees at the time.

The declassified record of U.S. documents shows an ambivalent American attitude toward the killings. American diplomats that summer urged restraint on southern officials - to no obvious effect - but a State Department cable that fall said overall commander Gen. Douglas MacArthur viewed the executions as a Korean "internal matter," even though he controlled South Korea's military.

Even educated South Koreans remained ignorant of their country's past. As a young researcher in the late 1980s, Yonsei University's Park Myung-lim, today a leading Korean War historian, was deeply shaken as he sought out confidential accounts of those days from ordinary Koreans.

"I cried," he said. "I felt, 'Oh, my goodness. Oh, Jesus. This was my country? It was true?' "

South Korean conservatives complain the "truth" campaign will only reopen old wounds from a time when, even at the village level, leftists and rightists carried out bloody reprisals against each other.

By exposing the truth of such episodes, "we hope to heal the trauma and pain of the bereaved families," the commission says. It also wants to educate people, "not just in Korea, but throughout the international community," to the reality of that long-ago conflict, to "prevent such a tragic war from reoccurring in the future." *