Al-Kassar was accused of trying to sell thousands of weapons to the Farc
A major Syrian-born arms dealer has been convicted in a US federal court in New York of conspiring to sell weapons to left-wing rebels in Colombia.
Prosecutors said Monzer al-Kassar was willing to supply thousands of weapons, including machine-guns, surface-to-air missiles and grenade launchers.
They said the Farc rebel group intended to use the weapons to kill Americans.
His Chilean co-defendant, Luis Felipe Moreno Godoy, was also found guilty. The two men could face life in prison.
The prosecution's case was based largely on evidence gathered by two undercover US Drug Enforcement Administration (DEA) agents, who posed as arms buyers for the Farc.
The agents filmed their negotiations in Spain with al-Kassar and Moreno Godoy.
'Prince of Marbella'
Al-Kassar was arrested at Madrid airport in June 2007.
The 62-year-old was extradited to the US this year after Spain received assurances from US authorities that he would face neither the death penalty nor a life sentence without chance of parole.
Defence lawyers maintained that the two men were in fact spying on behalf of Spanish intelligence during their dealings with the DEA agents. Both men plan to appeal.
Al-Kassar, a long-time resident of Spain, has been dubbed the "Prince of Marbella" for his glamorous lifestyle.
He was described by prosecutors at the trial as one of the world's most prolific arms dealers.
The US Embassy in Madrid said al-Kassar had been selling weapons since the 1970s to the Palestinian Liberation Front and clients in Nicaragua, Bosnia, Croatia, Iran, Iraq and Somalia.
In 2006, he was named by the Iraqi government as one of its most wanted men for allegedly helping to arm insurgents. A UN report meanwhile called him an "international embargo buster".
He was acquitted in 1995 of supplying arms that were used in the 1985 hijacking of the Achille Lauro cruise ship off the coast of Egypt, which resulted in the death of an American, Leon Klinghoffer.