A man holds a Colombian flag with a sign that reads "No more FARC" on Wednesday, November 23, in Cali, Colombia.
NEW: A fifth hostage is found alive, Colombia's defense minister says
NEW: President Juan Manuel Santos calls the killings a "crime against humanity"
The victims were reportedly shot in the head and the back
A military operation earlier this month killed former FARC head Alfonso Cano
(CNN) -- Colombia's main leftist rebel group shot and killed four hostages held for more than a decade, President Juan Manuel Santos said Saturday, vowing to fight the rebels with everything in reach.
A fifth hostage, a policeman, was found alive, Defense Minister Juan Carlos Pinzon told reporters.
"This is yet another example of how brutal and cruel the FARC is. ... When faced with security forces, they (the rebels) had no qualms about killing them in cold blood," Santos said.
The Revolutionary Armed Forces of Colombia, known as FARC, has been at war with the Colombian government since the 1960s. While severely weakened in recent years, the guerrilla group has continued to carry out kidnappings and attack security forces.
Chains were found near the bodies of the four hostages, who were kidnapped "more than 12 or 13 years" ago, Santos said.
The president called the killings a "crime against humanity" and swore their only effect would be to make police and soldiers more determined to fight "with everything in reach."
The bodies of the four men, all of whom were security force members, were found in the morning, according to Pinzon.
They were executed in the southern region of Caqueta, where the military was conducting operations against the rebel group, he said.
Three of the hostages were shot in the head, while the fourth was shot in the back, the defense minister said.
The fifth hostage, identified as Luis Alberto Erazo, had been held for roughly 12 years, Pinzon said. He reportedly ran from the rebel camp at the start of a firefight between rebels and Colombian troops. His condition is "acceptable," Pinzon said.
Earlier this month, a military operation killed then-FARC leader Alfonso Cano. Following his death, the FARC released a statement in which its leaders said they would not end their guerrilla struggle.