Gunmen on Wednesday killed six police officers in a northern Mexican state where rival cartels are fighting over drug trafficking routes, authorities said.Violence has spiked in recent years in Zacatecas, with experts citing a turf war between the Sinaloa and the Jalisco New Generation cartels for control of lucrative drug smuggling routes to the United States.The Department of Justice considers the Jalisco cartel to be "one of the five most dangerous transnational criminal organizations in the world."In January, 10 bodies were found in a sports utility vehicle abandoned in front of the governor's palace in the Zacatecas state capital.Across the country, more than 340,000 people have been killed in a wave of bloodshed since the government deployed the army to fight drug cartels in 2006."