Several border states are also addressing the issue, amid growing concern about possible cross-border impacts of the violence in Mexico, which claimed more than 5,300 lives last year -- double the number in 2007. The violence has included videos of torture and executions posted on the Web -- a tactic former officials say was likely copied from Islamic terrorists -- and hours-long firefights between authorities and criminals toting large-caliber automatic weapons. A State Department travel advisory for U.S. citizens last week compared these incidents to "small-unit combat."
The DHS plan, which "does not change or supersede any existing authorities … addresses how a number of government agencies would deploy federal resources to help state and local partners on the ground if local resources were overwhelmed," DHS spokeswoman Amy Kudwa told United Press International. She said the agencies would include the U.S. military "as needed."
"We have been coordinating with the Department of Defense," she said.
The Department of Defense "is aware of the DHS plan," said spokesman Lt. Col. Almarah Belk. The department "has provided some information on potential DoD-unique resources/capabilities -- based on historical precedent -- that could be employed in support of the DHS plan if asked."
"We haven't committed any resources/troops and there has not been a request for anything," Belk continued, adding that any requests would need approval from either the president or the secretary of defense to be granted.
Neither Belk nor Kudwa would give specifics, but non-lethal military capabilities like air transport are used in federal disaster response.
Kudwa said the DHS operations plan had been drawn up last summer to address "a broad spectrum of contingencies," including the possibility that the violence in Mexico might "spill over the border in such a way that it exceeds the capacity of federal, state and local law enforcement on the ground to respond."
News of the plan, first revealed last week by new Homeland Security Secretary Janet Napolitano and reported by Fox News, comes on the heels of the State Department warning for Americans planning to visit Mexico and a report from the U.S. National Drug Intelligence Center that found that Mexican cartels "maintain drug distribution networks or supply drugs to distributors in at least 230 U.S. cities."
Several southern border states are also addressing the issue, which was one of a number raised with Napolitano by California GOP Gov. Arnold Schwarzenegger in a meeting Monday, according to state officials.
"This is on our radar," Jay Alan, a spokesman for the California Emergency Management Agency, told UPI. He said the question had come up "not necessarily in a formal way" during discussions among the 10 states from both sides of the border -- four U.S., six Mexican -- who were negotiating a memorandum of understanding on emergency management.
In Arizona, Lt. James Warriner, a spokesman for the Department of Public Safety, told UPI the violence was "definitely creeping across the border into Arizona" in the form of military-style home invasions, kidnappings and human trafficking operations.
"There have been home invasions … using the tactics" familiar to law enforcement because of their employment by Mexican cartels. "They are very heavily armed" and sometimes wear paramilitary-style uniforms, Warriner said.
The more extreme manifestations of the violence -- beheadings and military-style firefights -- have not been seen in the United States.
Former White House drug czar John P. Walters told UPI some tactics may have been copied from Islamic terror groups. "The sequence and timing is suggestive," he said, pointing out the Mexican cartels had adopted the practice of circulating and posting videos of murders and torture after the tactic had been used by terror groups in the Middle East.
"The gruesome use of these technologies … to instill terror" was a recent development, Walters said.
Analysts say the extreme nature of the violence in Mexico is in part a product of the destabilizing effects of President Felipe Calderon's use of the army and federal authorities to crack down on the cartels -- which has provoked shootouts with police and escalating violence between the cartels as markets are destabilized by the removal of key players.
"Drug markets in general are not so violent," said Vanda Felbab-Brown, an expert at the Brookings Institution on the national security implications of illicit economies. "The drug market in Mexico is an aberration," she said, noting that the violence "imposes very high transaction costs on the drug trade itself."
Experts generally dismiss the idea that the Mexican state might collapse but do see the danger that the government's authority might become increasingly attenuated in some areas of the country.
"Mexico is nowhere near where Colombia was in 1999, when drug (dealers) and terrorists controlled 70 percent of the countryside and people said it was on the brink of being a failed state," Stephen Johnson, former deputy assistant secretary of defense for the Western Hemisphere, told UPI.
But other experts see the possibility that the government there might effectively withdraw or be forced out of certain areas, creating mini narco-states where its writ does not run.
If the Mexican government "retreats into tacit agreements with key drug cartels to adopt a live-and-let-live attitude," said Ray Walser of the Heritage Foundation, allowing local corruption to flourish unchallenged, "parts of Mexico could become no-go zones" for law enforcement, he said.
"The corruptive influence and increasing violence of Mexican drug cartels impedes Mexico City's ability to govern parts of its country," Director of National Intelligence Adm. Dennis C. Blair told the Senate Intelligence Committee earlier this month.