For some time, the United States has suspected that the Colombian Army tolerates paramilitaries - right-wing armies funded by large land-owners and drug traffickers.
Now, the military is strongly implicated in two massacres in the south. The killings are among the main reasons the US is holding up tens of millions of dollars in aid, and analysts say the Clinton administration's plan to aid only units with clean records may have been foiled.
"We are extremely concerned about this suggestion of tacit, if not active, support of paramilitary atrocities by the Army," wrote Barbara Larkin, US assistant secretary of legislative affairs in a Dec. 8 letter to Sen. Patrick Leahy (D) of Vermont.
The US State Department made its concern public in last year's human rights report. The Clinton administration sought to avoid the issue by giving aid only to units which operate in the country's south, according to Robin Kirk of Human Rights Watch/Americas in Washington.
Colombia's south, where coca - the plant used to make cocaine - is grown and processed, has no history of paramilitary activity.
The situation changed last July, when right-wing paramilitaries made their first of two attacks on civilians in the south.
"At this point in time I don't think the US can safely fund the Colombian military," says Ms. Kirk.
US aid to Colombia is primarily for antidrug operations, and the money has flowed steadily in recent years despite cutbacks on aid destined for Colombia's 33-year war with leftist guerrillas.
All aid specifically slated for the Colombian Army is currently being held up in an ongoing human rights screening process, but aid to the police continues, as do subsidized military sales.
Between direct aid, regional aid, and defense draw-downs - all subject to different regulations - the result is a confusing maze of figures.
"I doubt anybody really knows how many different programs result in the transfer of military equipment and assistence to Colombia," says Carlos Salinas, Latin America program officer for Amnesty International in Washington.
The first of the two massacres took place in the town of Mapiripan in the coca-growing region of Guaviare.
More than 100 heavily armed men flew into a military airstrip in San Jos. The Army denies any knowledge of the flight, as do the antinarcotics police. Next to the airstrip is the police barracks that houses a team of US advisers and pilots who are part of Colombia's drug-fumigation program.
There the "paras" took speedboats down river to Mapiripan. At least 26 residents were murdered over five days, many of them tortured and cast into the river, according to a report by Bogot human rights groups. In interviews this fall, Colombia's most powerful paramilitary warlord, Carlos Castao, took credit for the murders. He said that those killed were guerrilla collaborators.
The second attack stands out more painfully for the US, though the casualties were fewer.
Six people were killed last October by paramilitaries here in the town of Miraflores. Just a day later US drug czar Barry McCaffrey was in the area praising Colombian antidrug efforts. Also significant is the presence of antinarcotics police at the time of massacre. Up till now they were considered to have an excellent human rights record.
"The people were terrorized and running to hide," says Hector Guavita, a jeep driver who witnessed the arrival of six paramilitary fighters in Miraflores.
He points out where three of the victims were executed in broad daylight, just 100 yards away - and in plain sight - of the Miraflores military base.
Residents say that the killers were taken directly to the military base when they arrived and that they moved freely about the town, communicating with walkie-talkies.
After three days, Colombian soldiers called from a public phone for a private plane to collect the "paras," Mr. Guavita says.
Miraflores is a jungle town of about 5,000 people who subsist by growing coca. Access to the town is primarily by air, and the antinarcotics police register everyone who steps off a plane.
Army and police officers in Miraflores say that it would be impossible for anyone to enter the town without the knowledge of the military, but both police and Army in the village denied knowledge of a paramilitary attack.
"The decision whether to cut off military aid in full or in part will be made after it is determined whether the Colombian government is fully prosecuting," says a Clinton administration official in Washington.
The general with jurisdiction over both massacres has been transferred, but no one has yet been prosecuted. Military officials are subject to their own justice system, which has been criticized as partial by US officials.
The commander of Colombia's armed forces, Gen. Jos Manuel Bonett, has repeatedly denied the military's institutional involvement with paramilitaries, though he said in a recent interview that informal collaboration may take place in "some isolated cases." The head of the police, Gen. Rosso Jos Serrano, was not available for comment.