A Mexican police officer working as a hit man for drug gangs killed one of his police colleagues on Wednesday in the violent border city of Ciudad Juarez, police said.

Francisco Ventura was shot and killed by gunmen in two sport utility vehicles as he drove home early on Wednesday in Ciudad Juarez across from El Paso, Texas. Federal police arrested three men following the shootout, including the police officer accused of leading the gunmen.

City police declined to give more details, but Ciudad Juarez Mayor Jose Reyes said the discovery of a police officer working as a hit man showed the urgent need for a "total cleansing" of the city's police force.

"We know there are officers who are in the pay of organized crime and that is why we need to flush out bad police," Reyes told reporters.

Despite the deployment of 3,000 troops and federal police in Ciudad Juarez this year, more than 550 people have been killed in drug violence in the city, Mexico's most lethal front in a drug war that has killed 1,900 people nationwide in 2008.

Daylight gun battles have erupted on city streets and buildings set on fire as Mexico's most-wanted man, Joaquin "Shorty" Guzman, fights drug baron Vicente Carrillo Fuentes, boss of the Juarez cartel, for control of Ciudad Juarez and its lucrative smuggling corridor into the United States.