According to Colombian officials, who was responsible for the massacre?

Colombian officials say at least 24 peasants have been killed in a massacre Authorities blame on leftist rebels.
   Authorities in Cordoba department, about 450 kilometers northwest of the capital Bogota, suspect the country’s largest leftist guerrilla group, the Revolutionary Armed Forces of Colombia, or FARC, carried out the attack. They say a number of the victims were beheaded.
   The region also is a stronghold for right-wing paramilitaries who are known for brutally killing peasants, they are suspected of sympathizing with leftist guerrillas. Colombia has endured nearly four decades of civil war among the army, guerrillas and paramilitaries.  The rebels and paramilitaries fight each other for control of the South American nation’s lucrative coca fields. Coca is the key ingredient in the illegal drug cocaine.
   The United States is providing Colombia with more than one billion dollars in antidrug aid.