BOGOTA - Colombian FARC rebels executed four members of the security forces during a botched mission to free them from a decade as hostages, the most violent act by the group since troops killed its leader Alfonso Cano this month.The Revolutionary Armed Forces of Colombia, which has a policy of killing hostages if troops approach their camps, shot three of the captives in the head and the fourth in the back, President Juan Manuel Santos said.The bodies were found in chains, he said."These heroes of Colombia sacrificed their lives trying to bring peace to Colombia," Santos said. "This is another demonstration of the FARC's cruelty ... It's an atrocious crime."One police sergeant who was also being held hostage by the FARC managed to escape and was found alive by the military on Saturday, Defense Minister Juan Carlos Pinzon said.