Carlos Castaño Gil is the founder of the Autodefensas Campesinas de Córdoba y Urabá (ACCU), a right-wing paramilitary organization in Colombia. Castaño founded this group after his father was killed by the Fuerzas Armadas Revolucionarias de Colombia (FARC).

Castaño later founded an umbrella organization of paramilitaries operating in Colombia known as the Autodefensas Unidas de Colombia (AUC). The AUC is currently fighting a brutal war against the FARC in which many cilvillians have been killed. The AUC has been accused by human rights organizations of committing atrocities, and it has openly admitted to its involvement in the drug trade. The AUC is listed by the US Department of State as a Foreign Terrorist Organization.

Castaño was convicted in absentia of the murder of journalist Jaime Garzón, and sentenced to 38 years in prison.

On September 24, 2002, the United States Department of Justice unsealed an indictment against Castaño which accused him of trafficking over 17 tons of cocaine into the United States. Castaño announced that he would give himself up for trial in the United States and would accept his participation in numerous crimes, though he resented his being personally linked to the drug trade.

Castaño had become isolated from the organization according to some observers, as he seemed to become relatively critical of the AUC's increasing association with narcotraffickers in recent years and was more willing to compromise with the Colombian state, and thus the remaining AUC commanders might have turned their backs on him.

Castaño apparently suffered an attempt on his life on April 16, 2004, presumably at the hands of either his own bodyguards, those of rival paramilitary troops, or perhaps even other entities altogether. Acting AUC commanders claimed to believe that there was an accidental exchange of gunfire between his bodyguards and a separate group of paramilitary fighters, but that he may still be alive and possibly in hiding.

Other independent sources within the group and among its dissident factions claim that he and his men were captured and tortured before being executed and then buried by order of other AUC top leaders (perhaps his own brother Vicente Castaño and/or another commander nicknamed "Don Berna"), who have become increasingly close to narcotraffickers and their trade. Colombian investigators found a makeshift grave and an unidentified body (yet apparently not Castaño's) near the supposed area of the events. Those same sources allege that the bodies of Castaño and his other companions were dug up and taken to other locations before the investigators could arrive.

The apparent death of the AUC co-founder remains in the air and has been the subject of wild and rampant speculation. One of the more exotic rumours (dating to June 1, 2004), states that unidentified diplomatic sources told the AFP agency that Castaño may have been spirited away to Israel, via Panama, with U.S. assistance. No specific reasoning or details regarding this claim have yet been produced. The U.S., Colombian and Israeli governments have separately denied this allegation, which remains unconfirmed by any other parties.

For now, sources from either the AUC or other local militant factions have continued to dispute the exact whereabouts of Carlos Castaño. His deeply-rooted involvement in the Colombian guerrilla conflict had given him enough time to establish personal and business connections between narcotrafficers and local representatives of legal money-making industries, which might allow for the possibility of their collaboration in his conspicious disappearance or murder. Despite these claims, the truth regarding Castaño's exact condition has yet to be revealed, as no body has ever been found and no specific hiding place has been uncovered.