MEXICO CITY – At least 21 people were killed in drug-related violence across Mexico over the weekend, with 15 murders occurring in the northern state of Chihuahua and six others in the southern state of Guerrero, prosecutors said Monday.

Seven people were gunned down in Chihuahua city and eight others in Ciudad Juarez, located across the border from El Paso, Texas.

A woman and three bus drivers were among those killed in Juarez, considered Mexico’s most dangerous city and the scene of frequent shootouts between rival drug traffickers.

One of the drivers, identified as 23-year-old Luis Cardona, was gunned down inside his bus as dozens of terrified passengers scrambled to escape through the vehicle’s rear door.

Two bullet-riddled bodies were found in Juarez, one in the city’s western section and the other in the Bosques de Waterfill district.

Several groups of unidentified individuals also set fire over the weekend to four businesses, which burned to the ground, bringing to 13 the number of businesses torched in Ciudad Juarez in recent days.

The attacks on the businesses, three of which sold spare parts for automobiles, were all carried out by gunmen who threw Molotov cocktails at the buildings, eyewitnesses said.

Six men in Guerrero state, meanwhile, were gunned down by a mysterious armed group calling itself “El jefe de jefes” (The Boss of Bosses).

The group, which only appeared in recent days, claimed responsibility last Friday for 11 other killings.

Messages from the group were found with the 17 bodies in Guerrero, while the press reported that six other bodies along with similar messages were found in the central state of Morelos on Friday.

The motive for the killings has not been determined, but the Gulf and Beltran Leyva cartels, which formed an alliance in the past year to battle the Sinaloa drug cartel, operate in Guerrero.

Mexico has been plagued in recent years by drug-related violence, with powerful cartels battling each other and the security forces, as rival gangs vie for control of lucrative smuggling and distribution routes.

Armed groups linked to Mexico’s drug cartels murdered around 1,500 people in 2006 and 2,700 people in 2007, with the 2008 death toll soaring to more than 6,000.

The death toll from drug-related violence, according to unofficial press tallies, stands at more than 5,600 this year.

Mexico’s most powerful drug trafficking organizations, according to experts, are the Tijuana cartel, which is run by the Arellano Felix family, and the Gulf, Juarez and Sinaloa cartels.

Two other large drug trafficking organizations, the Colima and Milenio cartels, also operate in the country.

“Los Zetas,” a group of army special forces veterans and deserters who initially worked as hitmen for the Gulf organization, may now be operating as a cartel, some experts say.

La Familia Michoacana, which operates in the western state of Michoacan, the southern state of Guerrero and the central state of Mexico, which surrounds the Federal District and forms part of the Mexico City metropolitan area, is considered the largest trafficker of synthetic drugs in Mexico.

The Sinaloa organization is the oldest cartel in Mexico and is led by Joaquin “El Chapo” (Shorty) Guzman, who was arrested in Guatemala in 1993 and pulled off a Hollywood-style jailbreak when he escaped from the Puente Grande maximum-security prison in the western state of Jalisco on Jan. 19, 2001.

Guzman, considered extremely violent, is one of the most-wanted criminals in Mexico and the United States, where the Drug Enforcement Administration has offered a reward of $5 million for him.

Since taking office in December 2006, President Felipe Calderon has deployed more than 45,000 soldiers and 20,000 federal police officers across Mexico in a bid to stem the wave of violence unleashed by drug traffickers.

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 prosecutors. EFE