What looked like a Mexican military patrol aiding drug traffickers on the border shocked Texas police.

It was hardly a relief to the U.S. when Mexico announced Wednesday that the men were imposters: It meant that gangs feel free to drive around the border area with military-style vehicles and uniforms

Mexico has become accustomed to traffickers disguised as cops or soldiers.

It's not just the uniforms; gangs in Mexico often use grenades and rocket launchers. The suspects in Monday's incident had a military-style Humvee.

Caps, vests and T-shirts with official-looking logos for Mexico's federal police are sold at street stands. Some cops even rent out their uniforms or patrol cars to shakedown artists.

"It's very easy to go out and buy military uniforms in a store," said Rodolfo Casillas, a professor who specialized in crime at the Latin American School for Social Sciences. "It's very easy to get (uniforms) for any police agency you want to imitate."

Rick Glancey of the Texas Border Sheriffs' Coalition said the confrontation began 50 miles east of El Paso when state police tried to stop three sport utility vehicles on Interstate 10. The vehicles made a quick U-turn and headed south toward the border, a few miles away.

Crossing the border, one SUV got stuck in the river, and the men with the Humvee tried in vain to tow it out. Then a group of men in civilian clothes began unloading what appeared to be bundles of marijuana, and set the SUV on fire before fleeing.

It was a tense confrontation at a time of rising anger over border security. The U.S. is considering extending a wall along the 2,000-mile border — something Mexicans bitterly resent.

U.S. Ambassador to Mexico Tony Garza issued a strongly worded statement late Wednesday, saying the incident "highlights the need for increased enforcement efforts by the United States and serves to bolster the arguments of those who seek the creation of physical barriers along our border."

Recent reports that Mexican soldiers and police have been crossing into the United States about 20 times a year have irked U.S. border states — even though Homeland Security Secretary Michael Chertoff downplayed the problem, noting that in many places the border is not clearly marked.

Mexican presidential spokesman Ruben Aguilar told a news conference Wednesday that "it is known that these are drug traffickers using military uniforms and they were not even regulation military uniforms."

Mexico also confirmed its longstanding policy that its soldiers must stay away from the border unless they have special authorization.

A U.S. law enforcement official in Washington, speaking on condition of anonymity because the matter is politically sensitive in both countries, confirmed Aguilar's account, saying the FBI and other agencies had found no evidence that the men in uniform were Mexican soldiers. State Department spokesman Sean McCormack said both governments were investigating.

Francisco Aguilar of the Mexican army press office said he had no information on Monday's incident.

Mexico's Foreign Relations Department said in a statement that Monday's incident, in which shots were not fired, could have been staged to "damage the image of our armed forces and bilateral cooperation."

Either way, few people along the border are likely to be reassured by the official confirmations that drug traffickers drive around the U.S.-Mexico border in military-style vehicles and dressed in military garb.

"We are in a much more sensitive situation because it could well be that these are people disguised as police," Casillas said. "Or it could be officers who are abusing their authority."

Mexico has struggled to remove corrupt law enforcers and keep security equipment out of the wrong hands. But police and soldiers have been arrested and charged with carrying out drug operations and even kidnappings.

Aguilar, the presidential spokesman, said fighting organized crime is "a long-term, short-term and medium-term battle that this administration faces, and that will also face the following administrations."

One of the most high-profile cases involves the "Zetas," a gang of deserters from an elite Mexican army military unit that has fought a bloody turf war to control trafficking routes on the border.