Ciudad Juarez, Mexico (CNN) -- 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.
"We are afraid because nobody, including the teachers, said anything to us. We learned about it because we heard about it from other mothers and nothing more ... we are afraid that something is going to happen to our children," mother Rosa Villagrana said.
Chihuahua state Gov. Cesar Duarte traveled to Juarez to speak out against the threats, which first appeared several weeks ago.
"We could not ever allow what is being signaled, even with the severity of the security crisis, but an attempt is being made to destroy the integrity and the tranquility of the teachers, the principals, the parents and the children," he said. "To the criminals we say that whoever dares to extort will face life imprisonment."
Prosecutors have not captured whoever is responsible for the messages or determined whether they are part of a prank.
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.
"People say that we have to entrust ourselves to God, right? Only he can say everything. Why can't the authorities? They no longer patrol and they no longer capture the bosses who are really behind this. They don't capture the bosses and they aren't patrolling anymore, so there are no results," father Oscar Olmos said.