The missing leader of Colombia's feared right-wing AUC paramilitaries was abducted and strangled to death by rivals, sources have reported.
Castano was feared throughout Colombia
Carlos Castano disappeared after a shootout at his ranch on 16 April and initial reports suggested he had gone into hiding from former comrades.
But a friend of Castano has told Reuters news agency the AUC leader was actually captured, tortured and killed.
His likely death could damage talks between the paramilitaries and Bogota.
Castano had been convicted of innumerable massacres and murders and was one of Colombia's most feared men.
At the time of the attack, the AUC (United Self-Defence Forces of Colombia) was in talks with the government to demobilise its 13,000 heavily armed fighters.
It seems that elements within the AUC had feared Castano was about to cut a deal with the US, where he is wanted on charges of smuggling 17 tonnes of cocaine.
The BBC's Jeremy McDermott reports that the attack on his ranch appears to have been carried out by other right-wing paramilitaries linked to the drugs trade.
The unnamed source who spoke to Reuters said he had spoken to witnesses who had seen what had happened at the ranch.
"They caught Commander Carlos alive that very day and they killed him two days later, after they kept him for two days with his hands and legs bound, in his underwear," the source said.
Responding to the Reuters report, Colombian President Alvaro Uribe said he was "respectful" of it but could not otherwise comment.
AFP news agency quoted a rival paramilitary leader, Commander Rodrigo "Double-Aught" Franco, as saying Castano had been killed for refusing to rule out the US extradition of his men on drug charges,
Our correspondent reports that Castano's death, if confirmed, endangers current peace talks between the government and the AUC which Castano began in December 2002.
Castano founded the AUC in 1997 although he had been fighting a war to the death with the country's left-wing rebels ever since they kidnapped and killed his father over 20 years ago.