CIUDAD JUAREZ, Mexico (AP) Gunmen killed a police officer and a jail guard Friday and left signs on their bodies saying they had fulfilled a promise to slay at least one officer every 48 hours until the Ciudad Juarez police chief resigns.

The slayings were a chilling sign that criminal gangs are determined to control the police force of Ciudad Juarez, a city of 1.3 million people across the Rio Grande from El Paso, Texas.

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

Police already were on "red alert" meaning they could not patrol alone after cardboard signs with handwritten messages appeared taped to the doors and windows of businesses Wednesday, warning that one officer would be killed every 48 hours if Public Safety Secretary Roberto Orduna does not quit.

Mayor Jose Reyes Ferriz insisted Friday that he would not back down.

"We will not allow the control of the police force to fall in the hands of criminal gangs," he said.

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 Ciudad Juarez, the largest city on the U.S.-Mexico border.

More than 50 of the dead are city police officers.

Portillo and city jail guard Juan Pablo Ruiz were killed as they left their homes before dawn to head to work, city spokesman Jaime Torres said.

Three days earlier, assailants fatally shot police operations director Sacramento Perez, the chief's right-hand man, and three other officers who were sitting with him in a patrol car near the U.S. consulate.

The bodies of Perez and one of the officers were sent to their home states Thursday to be buried and the city planned to hold a ceremony Friday for the two others from Ciudad Juarez.

City spokesman Jaime Torres said police have been asked to patrol with their guns in their hands.

Reyes Ferriz earlier ordered police to travel in groups of three patrol cars, with two officers in each vehicle.

Orduna has not spoken publicly since the threats. A retired army major, he took over as chief in May after former Public Safety Secretary Guillermo Prieto resigned and fled to El Paso following the slaying of his operations director.

For Orduna's protection, the city has built his bedroom at the police station so he does not have to go home. He also travels in different vehicles when he does go out.

Copyright 2009 The Associated Press.