By Robert Plummer
Of all the unresolved issues from the dark days of military rule in Latin America, Operation Condor is among the most sinister.
As many as six South American regimes took part in the joint campaign to hunt down and kill their left-wing opponents.
Although the conspiracy now dates back nearly 30 years, the consequences continue to cast a shadow over the present-day governments of the region.
A Chilean court has now ruled that former Chilean President Augusto Pinochet is not mentally fit to be prosecuted over the operation.
But two other ex-leaders in the region are still being pursued by judges on related charges, as efforts continue to find out exactly who was responsible.
Operation Condor was founded in secret and remained a mystery until after democracy had returned to South America.
According to documents later discovered in Paraguay, it was established at a military intelligence meeting in Chile on 25 November 1975 - Gen Pinochet's 60th birthday.
Delegates from five other countries were there: Argentina, Brazil, Bolivia, Paraguay and Uruguay.
Following that meeting, the military governments of those nations agreed to co-operate in sending teams into other countries to track, monitor and kill their political opponents.
A joint information centre was established at the headquarters of the Chilean secret police, the Dina, in Santiago.
As a result, many left-wing opponents of military regimes in the region who had fled to neighbouring countries found themselves hunted down in exile.
But this transnational pact apparently went far beyond Latin America, with agents of Operation Condor accused of murder plots in various other countries, including Italy and the United States.
One high-profile killing associated with Operation Condor is the assassination of former Chilean Foreign Minister Orlando Letelier, who died in a car bomb explosion in Washington in September 1976, three years after the government in which he served was overthrown by Gen Pinochet.
The head of the Dina at the time, Manuel Contreras, was given a seven-year jail sentence in 1993 by a court in Chile for his role in Mr Letelier's death.
Gen Stroessner remains beyond the reach of Paraguayan justice
Contreras is now back behind bars, after he was sentenced to 15 years in May this year for involvement in another killing.
Contreras reported directly to Gen Pinochet for the duration of Chile's military government.
However, Gen Pinochet denied all knowledge of Operation Condor when he was questioned by a Chilean judge.
"No, I don't remember, because it wasn't my problem," he told Juan Guzman in September 2004. "That was an issue, I imagine, for mid-level officials."
Operation Condor might never have come to light at all but for a chance discovery in Paraguay in December 1992.
A local judge went looking for files on a former political prisoner at a police station in the capital, Asuncion - but instead he found detailed documents that have since been dubbed the Archives of Terror.
These contained information on hundreds of Latin Americans who had been secretly kidnapped, tortured and killed by the secret services of the military regimes involved.
Paraguay's former military ruler, Gen Alfredo Stroessner, has been charged in connection with disappearances related to Operation Condor.
However, he remains in exile in Brazil, where he is safe from prosecution.
The former head of the Paraguayan armed forces, Gen Alejandro Fretes Davalos, was charged in September with the same offences.
Gen Videla is facing other charges linked to Argentina's 'dirty war'
Under questioning, he too denied all knowledge of Operation Condor and said it was purely a police matter.
Responsibility for the offences, he said, lay with senior intelligence and police officials in Paraguay - both of whom, conveniently, are now dead.
In Argentina, efforts are also continuing to delve into the origins of Operation Condor - so far, with just as little success.
Gen Jorge Rafael Videla, who led the country's military junta from 1976 to 1981, is under house arrest in connection with separate inquiries involving accusations of illegal adoptions of children born to women detainees.
However, he has refused to give evidence to a judge who is leading investigations into Operation Condor in Argentina, and attempts to try him for involvement have so far come to nothing.