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The body of Miss Cubas was found last month in a shallow grave near Asuncion. Investigators discovered e-mails exchanged between the kidnappers and a FARC commander.

The Colombian guerrilla group denies involvement in the kidnapping. But the case prompted Colombia and Paraguay to announce this month a joint effort to battle narcotics trafficking and address Paraguay’s contention that FARC had infiltrated its borders.

U.S. officials and observers also contend that FARC is expanding its support of regional movements like the “cocaleros” (small-scale coca farmers) of Bolivia and remnants of the Shining Path in Peru, while cultivating drug trafficking links in and beyond the triple border region of northern Argentina, Paraguay and Brazil.

“The Paraguay case should not have come as a big surprise,” said Tom Marks, a political risk consultant. He said U.S. officials have had evidence as early as 2001 of the FARC aiding illegal activity throughout the region.

Mr. Marks said the group has taken a beating from Colombian military forces and turned more toward terrorism and guerrilla warfare.