"Having just toured several sections of the border in question, it is clear that there is no chance whatsoever that the army units were simply lost, or unaware that they had crossed the border," Mr. Tancredo wrote in the letter.
The Mexican government has in the past said that military and police agents, like many illegal migrants trying to cross the vast border area, have become lost during patrols.
The letter in response to Mr. Tancredo from Mexico's ambassador to the United States, Juan Jose Bremer, said that "the excellent level of political dialogue that currently exists between the governments of Mexico and the United States has allowed every case of supposed unnoticed or accidental crossings of Mexican or U.S. personnel into the territory of the other country to fully identify the circumstances."
"These are dealt with on a local level," said a Mexican government official, speaking on the condition of anonymity. "Every time we have an authority crossing to the other side, both governments exchange views of what happened."
There have been several Mexican military and police incursions in the past two years that have angered Mr. Tancredo. In March, four Mexican soldiers carrying submachine guns and automatic rifles were detained when they ventured into the United States and encountered a Border Patrol agent. In October 2000, 10 similarly armed Mexican soldiers were reported to have fired on a Border Patrol air unit after taking a position on the U.S. side near Copper Canyon in California.
In March 2000, Border Patrol agents in El Paso, Texas, said that two Mexican army Humvees, reported by the Mexican government to be on an anti-drug mission, crossed the U.S. border. Two shots were fired from one of the Mexican vehicles, agents reported, but no one was hit. One vehicle retreated into Mexico, but the nine soldiers riding in the second vehicle were detained temporarily before being returned to their country.
"These situations can be very difficult," said Keith Weeks, a patrol agent and vice president of Local 1613 of the National Border Patrol Council in California. "We are outgunned in these instances. They have automatic rifles, and we have handguns." He said that military assistance for drug running in Mexico is "a definite possibility."
The area Mr. Tancredo examined on his recent visit is filled with encampments where illegal immigrants stay during their journey north. Several towns near the border on the U.S. side have street signs exclusively in Spanish, and border lines are absent in some areas.
But travelers in the area are able to see markers designating U.S. territory in many places, especially in Coronado National Forest.
For several years, both sides have advocated better marking of the 2,000-mile Southwest border, but with no result. In some areas, trampled fences are all that remain of the former border marking, while some areas in New Mexico and Texas have 6-foot-tall monuments as signposts.
But Johnny Williams, who heads field operations for the Immigration and Naturalization Service, said that incursions along the border are inevitable given the tense situation. Some degree of corruption on both sides may be unavoidable.
"Vicente Fox has done a good job in rooting out corruption," Mr. Williams said. "I would not discount the fact that with thousands of officers on both sides, there may be someone in uniform doing something wrong."