Ciudad Juarez, Chihuahua - Juárez police went on alert Thursday and increased security at police stations after a threat was left at the city's police memorial following the killing of an off-duty officer.
The threat was found Wednesday after patrol officer Luis Alberto Adame Rubio was chased down in his car and fatally shot by a group of gunmen.
Adame, 32, joined the Juárez police a year ago as part of classes of recruits intended to weed out corruption.
The threat at the police memorial referred to Adame's murder and mentioned that the same thing would happen to police "who do not cooperate with La Linea," or the Juárez drug cartel.
Mexican police have historically faced a dilemma known as "plata o plomo" (silver or lead), in which police either take bribes from narco-traffickers or are killed.
City officials said the murder was the first attack on municipal police this year.
Tension grew late Wednesday night when gunshots were fired outside the Chihuahua district police station in southwestern Juárez.
Police spokesman Jacinto Segura denied reports that claimed the gunfire was a direct attack on the police station. Segura said there was no attack but there was a large police response after gunshots were heard on the street in front of the station. There were no injuries.
The killings continued on Thursday with at least nine homicides in the Juárez area.
Chihuahua state police said four men were found shot and killed Thursday morning near a ranch in the village of El Porvenir.
The body of Julio Cesar Murillo Orozco, 28, was found on a road in Rancho Rinconada Gallegos. Jose Luis Gonzalez, 28, was found 500 meters away in an irrigation canal, and two unidentified men were found one atop the other alongside the Juárez-Porvenir highway.
About 5:30 p.m., Abel Herrera Picasso, 39, was fatally shot in the San Isidro hardware store in the village of San Isidro, police said.
El Porvenir and San Isidro are in the Valley of Juárez. El Porvenir is across the border from the Fort Hancock area, and San Isidro is across from the San Elizario area.
The Valley of Juárez has been a hot spot as narco-traffickers fight for control of smuggling routes into the United States.
Also on Thursday, security remained increased for Juárez Mayor Jose Reyes Ferriz who was the target of a death threat found Wednes day along with a pig's head. The threat stated the mayor had two weeks left to live.
Reyes also has a home in West El Paso. El Paso police spokesman Detective Mike Baranyay said police were aware of the threat against Reyes but decline to discuss whether anything was being done in El Paso about the mayor's safety.