Four police officers and two civilians were gunned down in different sections of Ciudad Juarez, Mexico's murder capital, a police spokesman said.

Gunmen opened fire at close range Wednesday on three Chihuahua state Attorney General's Office investigators who were riding in an automobile, killing all of them, Juarez Municipal Police Department spokesman Adrian Sanchez said.

The gunmen walked up to the vehicle, which was parked outside a high school, catching the agents by surprise, investigators said.

A suspect was arrested after the attack, but it is not clear whether he was involved, Sanchez said.

A municipal police officer was gunned down in a supermarket parking lot in the southeastern section of Ciudad Juarez, which is across the Rio Grande from El Paso, Texas.

Two men were shot dead at about the same time at a business in downtown Juarez, a police spokesman said.

The victims, according to preliminary reports, were the manager and an employee of a telecommunications business.

The killings occurred on the eve of President Felipe Calderon's visit to Ciudad Juarez for the start of the "Juarez Competitiva" conference, which will examine ways of changing the image of the violent border city.

Ciudad Juarez has been plagued by drug-related violence for years.

The murder rate took off in the border city of 1.5 million people in 2007, when 310 people were killed, then it more than tripled to 1,607 in 2008, according to Chihuahua AG's office figures, with the number of killings climbing to 2,754 in 2009.

More than 3,100 people were murdered in the border city last year, making 2010 the worst year since a war between rival drug gangs sent the homicide rate skyrocketing in 2008.

The killing has not slowed this year, with more than 1,100 people murdered in Juarez.

The violence has not subsided in Ciudad Juarez despite the deployment of nearly 10,000 soldiers and Federal Police officers in the border city.

Chihuahua has accounted for nearly 30 percent of the more than 40,000 deaths registered in Mexico since Calderon declared war on the country's drug cartels shortly after taking office in December 2006.

The violence is blamed on a war for control of smuggling routes into the United States being waged by the Juarez and Sinaloa cartels with backing from hitmen from local street gangs.

Calderon has deployed tens of thousands of soldiers and Federal Police officers across the country to combat drug cartels and other criminal organizations.

The anti-drug operation, however, has failed to put a dent in the violence due, according to experts, to drug cartels' ability to buy off the police and even high-ranking officials.