A former leftist rebel who terrorized residents of southern Colombia after abandoning the country's peace process and plunging into drug trafficking has been killed in combat, Colombian President Ivan Duque announced Friday.

Walter Arizala was killed during a "heroic operation" earlier in the day after a months-long manhunt that followed the kidnapping and murder of three Ecuadorian press workers in the turbulent border area.

Many Colombians "can now sleep peacefully because one of the most horrendous criminals our country has ever known has fallen," Duque said.

Arizala, who is better known by his alias Guacho, led a small holdout faction of a few dozen guerrillas from the former Revolutionary Armed Forces of Colombia.

His group has been behind a wave of drug-related violence, including the March kidnapping of three newspaper workers from Ecuador's El Comercio newspaper who were later found slain.

Arizala's troops also are believed to have been behind the slaying of three Colombian judicial workers in July.

Duque provided no details about how Arizala was killed.

But chief prosecutor Nestor Martinez released a blurry photo of Arizala, smiling and wearing a baseball cap, that he said was taken during an undercover surveillance operation carried out in recent weeks by the military and judicial police.

Arizala's Oliver Sinisterra Front has its base of operations in Colombia's volatile Narino state, which is home to Colombia's largest harvest of illegal coca crops. Land dedicated to production of the raw ingredient used to make cocaine skyrocketed last year to 807 square miles (209,000 hectares) — the largest amount on record, according to a recent White House report.

Arizala, 29, was a mid-range FARC commander who joined the rebel group as a teenager. He initially joined in the peace process aimed at ending a half-century of political warfare in Colombia, but deserted and returned to the jungle last year. Authorities attribute at least 15 deaths to the group.