CIUDAD JUAREZ, Mexico (AP) - Ciudad Juarez's police chief stepped down Friday after criminal gangs made a chilling demand: Resign or we will kill more local officials.

Public Safety Secretary Roberto Orduna announced he was leaving his post only hours after gunmen killed a police officer and a jail guard. The gunmen left signs on thebodies saying they had fulfilled a promise made to slay at least one officer every 48 hours until Orduna quits.

The slayings were a grim sign that criminal gangs are determined to control the police force of the biggest Mexican border city. Juarez has a population of 1.3 million people and sits across from El Paso, Texas.

The chief's resignation was effective immediately.  Authorities said an interim chief would be named later.

Gunmen killed a police officer and a jail guard in the bloody border city of Ciudad Juarez, Mexico, today.

Officer Cesar Ivan Portillo was the fifth officer killed this week in Mexico's deadliest city.

The gunmen left signs on his and city jail guard Juan Pablo Ruiz's bodies, saying they'd fulfilled a promise to kill at least one officer every 48 hours until the Juarez police chief resigns.

Police already were on "red alert" in Juarez, meaning they can't patrol alone. Juarez Mayor Jose Reyes Ferriz insists that he won't back away from supporting Public Safety Secretary Roberto Orduna.

More than 6,000 people have been killed in drug violence across Mexico over the past year as gangs battle each other for territory and to fight off a nationwide crackdown by the army. Nearly a third of the slayings have taken place in Juarez, which is the largest city on the U.S.-Mexico border.

More than 50 of the dead are city police officers.

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