The system is failing Mexican journalists. Here’s how they’re fighting back.

A vanilla-infused breeze causes fragile palms to sway and wave along the streets of Papantla, a municipality in northern Veracruz, Mexico. Other than the occasional passerby photographing mural-covered buildings or the buzz of a motorcyclist emitting puffs of exhaust, this quiet town in the country’s eastern region is relatively at peace.

But on March 30, those peaceful streets turned red with blood when reporter María Elena Ferral Hernández was gunned down in broad daylight. Her death in a hospital a few hours later made her the first periodista murdered this...