Temperatures rise on Mexican border.

The deaths of 18-year-old Mexican Guillermo Martinez Rodriquez and Ismael Segura, 23, did not make national headlines in the United States.

Not so in Mexico and a few U.S. border towns.

The alleged shootings of the pair in separate incidents by U.S. Border Patrol agents sparked a furor in their native land.

As Congress and the Bush administration continue to display an increased determination to gain control of the border, tensions appear to be rising in southwestern states and in the respective nations' capitals.

Mexican media and officials accuse the agent of shooting Martinez in the back as he retreated across the border near San Ysidro, Calif. Martinez later died of a gunshot wound in Tijuana. Segura was allegedly shot in the chest less than a month later in Texas as he fled back across the border in a car.

Department of Homeland Security officials have acknowledged little about the San Ysidro incident other than the agent fired at Martinez after he threw rocks.

The incidents sparked a diplomatic row between the United States and Mexico. President Vicente Fox compared the proposed fence to the Berlin Wall. U.S. Ambassador to Mexico Tony Garza fired back that such comparisons were "disingenuous and intellectually dishonest," according to Reuters.

Fox sent a letter of protest to Washington, and Secretary of State Condoleeza Rice promised a full investigation. Meanwhile, Mexican media have pointed to the cases as signs of an increasingly militarized border.

With proposals in Congress to build high-tech fences, concerns are growing on the U.S. side that violent gangs who profit from smuggling drugs and illegal immigrants will push back.

Department of Homeland Defense Secretary Michael Chertoff said there was an "up tick" in violence directed at agents in 2005. "That is frankly one of the metrics we use to show that we are being successful in our interdiction."

Along the Rio Grande River in Texas, Border Patrol boats came under fire twice within a week earlier this year. Agents reported that highly skilled marksmen, able to hit a moving target under cover of darkness, unloaded dozens of rounds, injuring no one, but striking one of the boats, according to the Brownsville Herald.

Without commenting on the specific shootings, Chertoff said agents do have to adhere to rules of engagement. When agents fire a weapon, investigations are routinely carried out, as is the case for every law enforcement agency.

"We do have to be prepared for the fact that, as we press hard on these criminal organizations, some of them will want to fight back," Chertoff said. "But we ought to make it very clear that it will not cause us to back off."