Mr. Areiza Arango was one of several witnesses for whom the Supreme Court ordered state protection. He had been expected to testify in a witness-tampering case involving the Uribe brothers and paramilitary groups.

After the killing, Álvaro Uribe published a message on Twitter calling Mr. Areiza Arango “a bandit” and saying he was now “a good dead man.” After criticism, he said that the message had been written by someone else and that he did not celebrate the death of others.

‘I’ll Take Care of Myself’

La Carolina Ranch sits in the rolling hills of Colombia’s Antioquia Province, a four-hour drive from the regional capital, Medellín, past pine forests and vistas overlooking the Andes. In the 1990s it was known for raising bulls bred for the country’s bullfights and for pastures where cows produced milk sent to nearby towns.

But the pastoral setting masked a sinister conflict.

Colombia’s civil war was raging and leftist rebels roamed the countryside, committing massacres and extorting money from local ranch hands. In 1983, the conflict hit home for the Uribe brothers: Their father was killed by rebels during a kidnapping attempt on the family ranch. Álvaro Uribe found the body.

By the mid-1990s, Álvaro Uribe had risen to become Antioquia’s governor. In that role, he promoted armed neighborhood watch groups that were later accused of massacring rebels and civilians alike. Mr. Uribe has said he disbanded the groups when the allegations emerged.

At Santiago Uribe’s ranch, a similar pattern of violence was said to emerge.

Wealthy businessmen in the area, tired of paying extortion money to the rebels, formed an organization known as the Twelve Apostles, according to residents and prosecutors. The group — it got its name because a local priest was one of the leaders — worked closely with local police officers who acted as both informants and hit men, residents said.

“They kept lists of who was next to be killed, and we were all considered rebels just for being peasants in the countryside,” said one former resident, Fernando Barrientos. His brother, Camilo, was killed by the Twelve Apostles in 1994, he said. Mr. Barrientos has filed a criminal case against Santiago Uribe for the death. Mr. Uribe was jailed after his arrest in 2016 and then released to await trial.