After 48 hours, Colombian guerrilla group the FARC reportedly released five oil contract workers who were kidnapped Friday near the Venezuela border.

The workers are employed by Codisa, a Colombian company contracted by multinational Occidental Petroleum in Colombia’s eastern Arauca department. They were kidnapped July 29 in the municipality of Arauquita, which registered Arauca’s highest kidnapping rate last year, with 10 cases.

According to El Nuevo Herald, authorities have said the Revolutionary Armed Forces of Colombia (Fuerzas Armadas Revolucionarias de Colombia – FARC) were behind the kidnapping, although rebel group the National Liberation Army (Ejercito Nacional de Liberacion – ELN) is also active in the department and has long relied on kidnap ransoms as a source of funds. The FARC are usually responsible for kidnappings in the municipalities of Arauquita, Fortul and Tame, while the ELN are typically active in Saravena, Tame and Arauca.

Arauca is one of Colombia’s most heavily militarized department, with about 8,200 soldiers deployed here. Much of the force is meant to patrol the border with Venezuela or protect the Caño Limon-Coveñas pipeline, frequently the target of guerrilla attacks.

Still, the increased military presence may explain why the rebels now prefer to hold hostages in Arauca for brief amounts of time. Sources in the area have told InSight Crime that kidnappings usually last 15 days maximum, and that the rebels usually take their captives across the border to Venezuela. Frequent military patrols force the rebels to stay mobile, discouraging them from building the kinds of long-term camps used to keep hostages imprisoned in the past.

The release of the Codisa workers comes soon after the freeing of another contract worker employed by Canadian multinational oil company Talisman. The Talisman hostage was kidnapped alongside 22 other workers in March. Especially in light of the FARC’s kidnapping of three Chinese oil workers and their translator in June, these attacks appear to be part of a concentrated effort by the FARC to increase their harassment of the Colombian oil sector.

Statistics from the Defense Ministry show that kidnapping appears to be on the rise in 2011, with 108 cases registered so far this year, compared to 93 during the first half of 2010.