MONTERREY, Mexico – A group of gunmen attacked a detention center in the northern Mexican state of Nuevo Leon, freeing 23 inmates and killing 2 federal agents guarding the facility.

State police said a group of more than 20 gunmen on Friday attacked the detention center in Escobedo, part of the Monterrey metropolitan area, and killed federal agents Roberto Gonzalez Barco, 33, and Ricardo Mota, 42.

They then proceeded to free 23 inmates. Among them were several former police officers in the town of Garcia – also part of the Monterrey metro area – suspected in the recent murder of that municipality’s police chief, Gen. Juan Arturo Esparza.

Eyewitnesses told reporters that the inmates freed in the attack were whisked away in at least five vehicles.

State investigators, federal police and army soldiers arrived on the scene after the attack, but thus far no arrests have been reported.

Earlier Friday, 13 people died in Monterrey in a series of clashes between Mexican army troops and dozens of gunmen.

The confrontation began in the Monterrey suburb of Juarez after soldiers went to a ranch where they suspected that kidnappers were holed up with several captives, a participant in the events said.

The troops were met with heavy fire from around 50 heavily armed men thought to be drug-cartel enforcers, the military source said.

Seven suspects were killed in the exchange and nine others taken into custody, but the rest fled in more than a dozen vehicles.

While making their escape, they ran into a contingent of 30 soldiers sent to support the first unit, and another battle ensued, the source said.

Five gunmen and one civilian died in the second clash, which took place at the intersection of two major highways. Six other people – three suspects and three motorists – were taken to hospitals with injuries, the military source said.

The remaining gunmen managed to get away.

Among the gunmen killed in the shootouts was Ricardo Almanza, alias “El Gori,” the chief suspect in the November slaying of Esparza. A 30-million-peso ($2.3 million) reward had been offered for information leading to his capture.

Monterrey is a major industrial city and the capital of Nuevo Leon, one of the battlegrounds in a turf war among Mexico’s powerful drug cartels.

In other possibly cartel-related violence Friday, eight people were killed in several attacks in the northern border city of Ciudad Juarez, Mexico’s murder capital, while three police were fatally shot in the southern state of Guerrero and two others died during an attack in Aguascalientes state, northwest of the capital.

Mexico has been plagued in recent years by drug-related violence, with powerful cartels battling each other and the security forces amid a struggle for control of smuggling and distribution routes.

The drug war is blamed for more than 16,000 deaths in the past three years, with the 2009 death toll at roughly 6,800.

Since taking office three years ago, President Felipe Calderon has deployed 50,000 army soldiers and 20,000 Federal Police to 12 of Mexico’s 31 states in a bid to crush the cartels, but the pace of killings has only accelerated.