CIUDAD JUAREZ - Gunmen killed a senior police homicide investigator in Ciudad Juarez on the U.S.-Mexico border, the fourth policeman killed in the city this week, an attorney general's office said on Wednesday.

State police commander Vidal Barraza, who was investigating the 600 drug murders in Ciudad Juarez this year, was shot on Tuesday night as he stepped out of his house to meet a friend, the Chihuahua state attorney general's office said.

Barraza was appointed Chihuahua state murder investigator in July. He was barefoot and wearing only a pair of shorts when he was shot in his driveway.

"Witnesses told us they heard rounds of gunfire and then a few single shots," said the spokesman in the Mexican city across the border from El Paso, Texas. He declined to give more details.

Despite the deployment of 3,000 troops and federal police in Ciudad Juarez this year, the city has become Mexico's most violent front in a drug war that has killed 1,900 people nationwide in 2008.

Gun battles have erupted on busy city streets and buildings have been 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.