The Colombian police heard in early May that a big deal was going down inside a gated luxury community southwest of Bogotá. On May 3 they followed Colombian suspects, two of whom turned out to be retired Colombian Army officers, to a house filled with twenty-nine metal crates of arms and 32,000 rounds of ammunition. The police were still taking inventory of the cache when two more suspects knocked on the door. The police arrested them, only to learn they were US soldiers. The Colombian police said the arms were bound for an illegal paramilitary group that the State Department considers to be both a drug-trafficking and a terrorist organization.

The community of Carmen de Apicalá, where the arms were found, is only a short drive from Colombia's Tolemaida military base, home to US Black Hawk helicopters and the place where US Special Forces train Colombian troops in combat skills. For convenience as well as security, many US military personnel and contractors rent condominiums in Carmen de Apicalá. "It's a lot of ammunition, and it's a very suspicious case," Colombia's police commander, Gen. Jorge Castro, told local radio. Colombian lawmakers in Bogotá said the US Ambassador, William Wood, should explain the circumstances to the Colombian Congress.

The State Department spokesman in Washington, Richard Boucher, denied that the arms were part of a secret US effort to arm Colombian paramilitaries. But he still refuses to say whether the arms are part of the unprecedented $3.3 billion in military aid the United States began sending in 2000 as part of Plan Colombia. The Colombian attorney general's office, which is now investigating the case, said that the arms had been diverted from US stockpiles. The Colombian television station RCN broadcast footage of arms with US markings.

The case comes at a time when the Colombian government, led by President Álvaro Uribe, is negotiating a broad amnesty for Colombian paramilitaries. Known by their supporters as "self-defense" groups, Colombian paramilitaries have long been responsible for most of the country's politically motivated massacres and murders, which often target peasants, trade unionists and students they suspect of supporting leftist guerrillas. The rightist paramilitaries have also long been accused of secretly collaborating with the military to carry out death squad crimes.

"I think that it's probably fair to say that there is [sic] some episodes of contact between Colombian military and these so-called self-defense forces," Roger Noriega, the senior State Department official for Latin America, told Congress during questioning eight days after the Bogotá arrests, adding that such "episodes" are against Colombian law and US policy. Yet, in nearly every region of the country, Colombian military officers of all ranks have been found to be secretly collaborating with rightist paramilitaries, and only a few have ever been seriously prosecuted.

The United States itself has long been ambivalent about Colombia's paramilitaries. Back in the 1960s the US military, according to its own documents, encouraged the Colombian military to organize rightist paramilitary forces to help fight leftist guerrillas. By the early 1980s, Colombian drug traffickers and large landowners together organized the paramilitaries into a national force to ward off kidnappings and other forms of extortion by leftist guerrillas. But by the end of the decade, the government had outlawed paramilitaries after one group trained by the late drug lord Pablo Escobar blew up a Colombian airliner.

The Colombian military soon found a new way to maintain contacts with illegal paramilitaries, however. In the fall of 1990, according to a letter from the Pentagon to Senator Patrick Leahy, the US military helped its Colombian counterpart make its intelligence networks "more efficient and effective." It was instructed, according to an April 1991 classified Colombian military order, to keep its operations "covert" and "compartmentalized," to use only "retired or active-duty Officers or Non-commissioned Officers" as liaisons, and not to put orders "in writing."

One new intelligence network killed at least fifty-seven people, including trade unionists, community leaders and a journalist, according to judicial testimony. But charges were dropped after most of the witnesses were either murdered or disappeared. In 2001 a former Colombian Army general, Rito Alejo del Rio, was arrested by Colombian authorities from the attorney general's office on charges that he allegedly collaborated with illegal paramilitaries. But these charges, too, were soon dismissed, and the country's top two civilian prosecutors fled the country.

Later that year (one day before 9/11, ironically), the US State Department finally put Colombia's largest paramilitary group, the United Self-Defense Forces of Colombia, or AUC, on its list of terrorist organizations. In 2002 US authorities announced that the AUC was implicated in trading drugs for arms with none other than Al Qaeda. US authorities finally began indicting more Colombian rightist paramilitary leaders on drug charges, after having already indicted Colombian leftist guerrilla leaders on drug charges.

The May arrests of two US military officers for allegedly running arms to AUC paramilitaries raises many questions. US warrant officer Allan Tanquary and Sgt. Jesus Hernandez are now back in the United States, where officials say they may face criminal charges. "We're committed," said spokesman Boucher, "to a full investigation."Frank Smyth

Frank Smyth is a freelance journalist and co-author of Colombia's Killer Networks: The Military-Paramilitary Partnership and the United States, published by Human Rights Watch.