A black Mercedes-Benz rolled into a working-class neighborhood in Medellín and stopped in front of a small, worn-down house.

The paint on the walls had begun to flake. Rusted bars guarded the windows. In the tiny front yard, a few struggling plants fought for space among the weeds.

A man around twenty-five stepped out of the car. He looked too polished for this street—tailored suit, neat hair, expensive shoes. In one hand, he carried a leather briefcase. In the other, a thick envelope. As he walked toward the wooden door, his fingers tightened around the envelope, and his breathing turned shallow. He hesitated, then rang the bell.