MEXICO CITY—The murder of a mother who spent two years trying to bring to justice the killer of her 16-year-old daughter has struck a nerve in Mexico, casting a harsh light on the country's dysfunctional judicial system.

Marisela Escobedo, 52, was a largely unknown figure until last week, when a man shot her in the head in front of the governor's office building in the capital of Chihuahua, a northern border state known for its drug-related violence. Weeks earlier, Ms. Escobedo had begun a sit-in across the street, refusing...