SEOUL, South Korea - In a political about-face, a South Korean commission investigating a century of human rights abuses has ruled that the U.S. military's large-scale killing of refugees during the Korean War, in case after case, arose out of military necessity.

Shutting down the inquiry into South Korea's hidden history, the commission also will leave unexplored scores of suspected mass graves believed to hold remains of tens of thousands of South Korean political detainees summarily executed by their own government early in the 1950-53 war, sometimes as U.S. officers watched.

The four-year-old Truth and Reconciliation Commission of Korea probed more deeply than any previous inquiry into the country's bloody past. But a shift to conservative national leadership changed the panel's political makeup this year and dampened its investigative zeal.

The families of 1950's victims wanted the work continued.

"The truth about all these past incidents must be revealed, so this national tragedy won't be repeated," said Yang Won-jin, 82, whose father was believed shot and dumped into a mass grave 60 years ago.

But the commission's new president said its work must end.

"Even if we investigated more, there's not much more to be revealed," said Lee Young-jo, a political science professor who took charge last December.

The commission was established in December 2005 under the late liberal President Roh Moo-hyun to "reconcile the past for the sake of national unity." It had a broad mandate to expose human rights abuses from Korea's pre-1945 Japanese colonial period through South Korea's military dictatorships into the 1980s.

The most shocking disclosures emerged from the war that began when communist North Korea invaded the south on June 25, 1950, to try to reunify the peninsula, divided into U.S.- and Soviet-occupied zones in 1945.

The commission was the first government authority to publicly confirm what long had only been whispered: The U.S.-allied South Korean military and police carried out a vast secretive slaughter of political detainees in mid-1950, to keep southern sympathizers from supporting the northerners. Up to 200,000 were killed, historians believe.

Hundreds of petitions to the commission told another story as well, of more than 200 incidents in which the U.S. military, warned about potential North Korean infiltrators in refugee groups, was said to have indiscriminately killed large numbers of innocent South Korean civilians in 1950-51.

Declassified U.S. documents uncovered over the past decade do, indeed, show commanders issuing blanket orders to shoot civilians during that period. In 2007-2009 the commission verified several such U.S. attacks, including the napalm-bombing of a cave jammed with refugees in eastern South Korea, which survivors said killed 360 people, and an air attack that killed 197 refugees gathered in a field in the far south.

The liberal-led commission, with no power to award reparations, recommended Seoul negotiate with the U.S. for compensation for survivors of what it agreed were indiscriminate attacks. But the government of President Lee Myung-bak, elected in December 2007, has taken no action.

Lee's Grand National Party had warned during his election campaign that the truth panel's work could damage the U.S.-South Korean alliance.

Late last year, expiring terms on the 15-member commission enabled the Lee government to appoint more sympathetic commissioners, who opted not to extend the body's life by two years and instead to shut it down on June 30. Lee, the new panel chief, withdrew from distribution a 2009 English-language report on commission findings.