Six of the police officers killed in Friday's attack were federal, and one was a local police woman, said Enrique Torres Valadez, a spokesman for the state of Chihuahua, where Ciudad Juarez is located. Two local police officers were in critical condition.
Authorities said the police officers had stopped to talk to a street vendor who flagged them down for help when gunmen opened fire from behind their pickup patrol trucks. The assailants fled in three vehicles.
Investigators said they don't know why the officers were shot, although they don't believe they were targeted because of any recent arrests they had made.
No one has been arrested but police said they have recovered two of the three cars used in the shooting.
Hours after the attack, a painted message directed to top federal police commanders and claiming responsibility for the attack appeared on a wall in downtown Ciudad Juarez. It was apparently signed by La Linea gang, the enforcement arm of the Juarez drug cartel. The Juarez cartel has been locked in a bloody turf battle with the Sinaloa cartel, led by Joaquin "El Chapo" Guzman.
"This will happen to you ... for being with El Chapo Guzman and to all the dirtbags who support him. Sincerely, La Linea," the message read. The authenticity of the message could not be independently verified.
Ciudad Juarez, a city of 1.3 million across from El Paso, Texas, is one of the world's deadliest cities, and a two-year turf battle between drug cartels has left more than 5,000 people dead.
Elsewhere, police in the Pacific coast state of Guerrero said they found the bodies of five men who had been shot to death lying on a dirt road near the state capital, Chilpancingo. Three of the men were brothers, all in their 20s.
The state has been a major battleground for warring cartels, including the Beltran Leyva gang, but it was not clear whether the shootings were part of the ongoing drug violence.
In central Morelos state, federal police and the Mexican army raided two ranch homes and arrested 15 men near the town of Amacuzac. Those arrested were taken to Mexico City in a helicopter.
The men are suspected of working for alleged drug trafficker Jose Gerardo Alvarez Vazquez, who was arrested on Wednesday in Mexico City, said Ramon Pequeno, the head of the anti-narcotics division of Mexico's federal police.
Authorities say Alvarez Vazquez has been battling for control of the Beltran Leyva drug cartel with his partner, Edgar Valdez Villarreal, a U.S.-born enforcer known as "La Barbie."
Pequeno said the men arrested provided security and carried out killings for Alvarez Vazquez and Valdez Villarreal.
In the western state of Michoacan late Friday, the mayor of a town arrested last year for alleged ties to drug traffickers was released from prison.
Genaro Guisar Valencia, who was stripped by lawmakers of his post as mayor of Apatzingan because of his arrest, told reporters outside the prison in the state capital of Morelia that he would ask the state's legislature to reverse its decision.
Guisar Valencia was among 12 Mexican mayors arrested last year in an unprecedented roundup of elected officials accused of protecting drug traffickers in Michoacan.
He's the ninth mayor released for lack of evidence.
An estimated 22,700 people have been killed in Mexico's drug war since December 2006.
By Associated Press Writer Olivia Torres