A series of recent graffiti messages on school walls in Mexico's most violent city threaten attacks if teachers don't hand over their Christmas bonuses

The ominous warnings have sent a new wave of fear through Ciudad Juarez, Mexico, where groups of concerned parents have pulled their children out of class, some schools have been evacuated and authorities say they are stepping up security. Chihuahua state Gov. Cesar Duarte traveled to Juarez to speak out against the threats. Prosecutors have not captured whoever is responsible for the messages. Authorities have created a special team to investigate extortion in Ciudad Juarez, located across the border from El Paso, Texas. But some parents say that does little to calm their fears.