MEDELLIN, Colombia -- Gen. Mario Montoya has for years been a trusted caretaker of the sizable aid package Washington provides Colombia's army, leading helicopter-and-commando teams that eradicated drug crops and helping orchestrate this summer's dramatic rescue of hostage Ingrid Betancourt and three captured U.S. defense contractors from Marxist rebels.

With his cinematic bluster and take-charge nature, he impressed visiting American congressional delegations and military officials as an effective, no-nonsense commander who produced results.

But now, a former paramilitary fighter has said in special judicial proceedings that Montoya, who heads Colombia's army, collaborated with death squads that took control of this city's poor neighborhoods from the guerrillas a few years ago. His testimony, along with that of at least four paramilitary commanders, is illuminating the links between Colombia's potent, U.S.-backed military and its brutal paramilitary proxies.

Already, Uribe's administration has been shaken by the arrest this month of a friend and ally of the president, retired Gen. Rito Alejo del Río, on murder charges. Investigators say del Río built a brutally effective counterinsurgency force with paramilitary militias and often planned joint operations with the top paramilitary leader of the late 1990s and early part of this decade, Carlos Castaño. Del Río denies ties to paramilitary groups.

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