The U.S. government knew that top Guatemalan officials it supported with arms and cash were behind the disappearance of thousands of people during a 36-year civil war, declassified documents obtained by a U.S. research institute show.

The National Security Archive, a Washington D.C.-based institute that requests and publishes declassified government documents, obtained diplomatic and intelligence reports from the U.S. State Department under the Freedom of Information Act and posted them on its Web site on Wednesday.

"Government security services have employed assassination to eliminate persons suspected of involvement with the guerrillas or who are otherwise left-wing in orientation," one 1984 State Department report said.

Guatemala's U.S.-backed army battled leftist guerrillas in a 1960-1996 civil war that left more than 200,000 people dead or missing. Most were Mayan Indians.

"The government is obviously rounding up people connected with the extreme left-wing labor movement for interrogation," then-U.S. Ambassador Frederic Chapin said in a 1984 cable.

Chapin also said he was optimistic that missing union activist Fernando Garcia was alive and would be released. But Garcia has never been found, and two police officers were arrested in his case last week based on information found in Guatemalan police documents discovered in 2005.

The U.S. and local police files show that disappearances and executions were part of a deliberate strategy to crush leftist rebels, said Jesse Franzblau, a researcher at the Archive.

Kidnappings became "institutionalized" as the insurgency swelled in the 1970s and peaked during 1983-1986 government of Oscar Mejia Victores, "particularly in urban areas and against labor leaders, students, academics and political opposition leaders," a 1986 State Department report said.

Many innocent people were wrongly accused of involvement by "vengeful neighbors or others eager to eliminate personal, business or political rivals by proxy," it added.

"The tactic apparently accomplished its intended purpose — by 1984 insurgent networks in the capital had been decimated."