The army commander of the Arauca department where the kidnapping took place told Caracol Radio that “because of the location from where they were taken, a region where the ELN is active, we assume that the ELN has them.”
According to the official, some 500 troops are looking for the missing oil workers.
Authorities have promised a $57,000 reward for information leading to the release of the kidnap victims.
The kidnapping has led to a number of contradictory statements by Colombian authorities. Initially, officials said nine oil workers were kidnapped, which was later corrected to 11. On Wednesday morning, Defense Minister Juan Carlos Pinzon announced the victims had been liberated, but was forced to retract that statement only hours later.
Rebel group FARC, who on Sunday had publicly renounced the kidnapping of civilians, was initially held responsible for the kidnapping.