Keep digging:

MEXICO CITY - Mexico's top diplomat suggested Thursday that American soldiers disguised as Mexican troops may have been in the military-style Humvee filmed earlier this week protecting a marijuana shipment on the border.

Foreign Secretary Luis Ernesto Derbez also told a news conference that U.S. soldiers had helped drug smugglers before. However, he offered no evidence.

The U.S. Embassy in Mexico made no immediate comment on Derbez's claims...

Derbez said Thursday that the men photographed by Texas law enforcement could have been Americans.

"Members of the U.S. Army have helped protect people who were processing and transporting drugs," Derbez said. "And just as that has happened ... it is very probable that something like that could have happened, that in reality they were members of some of their groups disguised as Mexican soldiers with Humvees."

Three U.S. soldiers have pleaded guilty to running a cocaine smuggling ring from a U.S. base in Colombia, and a fourth is being tried in Texas this week.

Derbez said there was no proof that the men seen in the incident were Mexicans.

Derbez also said his country will send a diplomatic note to Secretary of State Condoleezza Rice demanding that U.S. officials tone down their comments on Mexico's security and immigration problems.

Meanwhile, the LAT has details on a few past incursions:

Several of the incidents described in the Department of Homeland Security reports appeared to involve Mexican officials getting lost or pursuing suspects. For example, five Tijuana police officers pursued two men across the border in 2004. Some of the officers fired at the suspects while on U.S. soil, according to a Border Patrol report. The police returned to Mexico after arresting the men.

Other encounters were more suspicious and add to concerns among many U.S. law enforcement officials that corruption in Mexico is eroding efforts to gain control of the border and combat trafficking in humans and drugs.

In October, Border Patrol agents in the El Cajon area east of San Diego reported seeing Humvees on the south side of the border fence. Minutes later, they saw two men in Mexican military uniforms carrying rifles in a creek bed north of the border, according to the records. When an agent approached, the two men ran south and drove off in the Humvees. Agents found footprints indicating three or four individuals had come north of the border and then returned.

Other incidents included Mexican helicopters flying north into U.S. airspace near El Paso for about 15 minutes, five Mexican officials armed with assault rifles entering the country near El Centro and returning without incident, and two Mexican police officers observed wandering along the U.S. side of the border near Yuma, Ariz.

Witnesses in El Paso reported in 2004 that a Mexican military-style helicopter landed just south of the border and armed men in federal police uniforms crossed into the U.S. and questioned them about vehicles before returning to Mexico, according to a Border Patrol report...

UPDATE: From this:

In a [formal] diplomatic note issued late Wednesday by the U.S. embassy in Mexico City, Ambassador Garza, requests a full investigation of the incident...

The U.S. Ambassador also protested what he refered to as rhetoric in Mexico that characterizes U.S. efforts to control its border as racist. He said that Mexican efforts to oppose measures under consideration in the U.S. Congress further polarize the debate on immigration and border security. He added that the Mexican actions, "only serve to polarize the debate on immigration and undermine the efforts of those who seek viable solutions to illegal immigration and border security."