After a half-century of war, peace has come to this long-troubled region of Colombia, and the change has been terrifying.

On Christmas Day, gunmen assassinated a rural activist from the leftist Marcha Patriótica party as he rode home on his motorbike. A member of the group was ambushed along the highway here in early November. The mutilated body of another activist turned up two weeks later in the same area.

The killings appear to fit a pattern of attacks on left-wing activists, indigenous leaders, human rights advocates and members of Marcha Patriótica, with the pace picking up in recent months as the government finalized a controversial peace accord with Marxist FARC rebels to end Latin America’s longest-running conflict.

By stalking grass-roots activists who are promoting the accord and pushing for its full implementation, the killers have sent a chill across the Colombian countryside and sown new doubts about the pact’s chances for success.