The warning to Border Patrol agents in Tucson, Ariz., comes after increased sightings of what authorities described as heavily armed Mexican military units on the U.S. side of the border. The warning asks the agents to report the size, activity, location, time and equipment of any units observed.
It also cautions agents to keep "a low profile," to use "cover and concealment" in approaching the Mexican units, to employ "shadows and camouflage" to conceal themselves and to "stay as quiet as possible."
Border Patrol spokesman Salvador Zamora confirmed that a "military incursion" warning was given to Tucson agents, but said it was designed to inform them how to react to any sightings of military and foreign police in this country and how to properly document any incursion.
T.J. Bonner, a 27-year Border Patrol veteran who heads the National Border Patrol Council said "Intrusions by the Mexican military to protect drug loads happen all the time and represent a significant threat to the agents.
"Why else would they be in the area, firing at federal agents in the United States? There is no other explanation," said Mr. Bonner, whose organization represents all 10,000 of the nonsupervisory Border Patrol agents.
He also challenged reports that Mexican military units had crossed mistakenly into the United States, saying, "Every country's military has a [global positioning system] nowadays, including the Mexicans.
"If the border is so poorly marked, why don't the thousands of Border Patrol agents working 24/7 along it ever seem to get lost, and none of us have been issued a GPS," he said.
A total of 216 incursions by suspected Mexican military units have been documented since 1996 -- 75 in California, 63 in Arizona and 78 in Texas, according to a Department of Homeland Security report.
Attacks on Border Patrol agents in the past few years have been attributed to current or former Mexican military personnel. U.S. law-enforcement officials have long thought that current and former Mexican soldiers are being paid to protect drug shipments bound for the United States.
Several agents said the attacks have escalated in the past two years as U.S. security efforts on the border have increased -- including the July shooting of two agents in an ambush near Nogales, Ariz., by assailants in black commando-type clothing, who fired more than 50 rounds. Authorities said the gunmen used military-style cover-and-concealment tactics to escape back into Mexico. No one has been arrested.
Santa Cruz County, Ariz., Sheriff Tony Estrada said that at least four shooters were involved and that his deputies found commando clothing, food, water and other "sophisticated equipment" at the site.
Several former Mexican soldiers trained in the U.S. as anti-drug commandos are now part of a well-armed gang known as the "Zetas," which has been linked to hundreds of killings and kidnappings on both sides of the U.S.-Mexico border in southeastern Texas. Many of the gang members have been identified as ex-members of an elite, anti-drug paratroop and intelligence battalion called the Special Air Mobile Force Group, who deserted in 1991.