Mexico City - Gunmen shot dead a US customs and immigration agent and wounded another on Tuesday in Mexico, where violence between powerful drug cartels and security forces has surged.
The two Immigration and Customs Enforcement (ICE) agents were driving north on Mexico's main highway on official business when they were attacked in broad daylight.
It was not immediately clear why they were targeted.
The US government condemned the attack, which came just over two weeks after Homeland Security Secretary Janet Napolitano warned Mexico's drug cartels not to take their violent tactics across the border.
“Any act of violence against our ICE personnel ... is an attack against all those who serve our nation and put their lives at risk for our safety,” Napolitano said in a statement after the agents were shot.
They were shot in the mid-afternoon south of the city of San Luis Potosi, which is roughly half way between Mexico City and Monterrey, the country's business capital where drug-related violence has soared over the past year.
The two agents may have been ambushed after stopping at what appeared to be a military checkpoint, said a Mexican official who declined to be named because he was not authorised to speak about the case.
Mexican drug cartels have been known to set up official-looking checkpoints, and the official said security forces had no checkpoints in the area.
Television footage showed a blue sports utility vehicle with several large bullet holes lying in the median of the highway, which was guarded by heavily armed Mexican federal police.
The US agents were rushed to a hospital where one died of his injuries. The second agent, who was shot in the arm and the leg, remains hospitalised, ICE said.
More than 15 000 people were killed in drug violence in Mexico last year but, despite growing domestic criticism of President Felipe Calderon's army-led strategy, the government has vowed to press on with its campaign to crush the cartels.
The violence has alarmed Washington, which worries that the fighting could spill over the border. It has also prompted some companies to reconsider plans to invest in Mexico.
The United States has provided funds and training to help Mexico in its fight against the cartels and intelligence from US law enforcement sources is credited with helping Mexico kill and capture several cartel leaders in recent years.
Attacks on Mexican police by drug gangs are common but US government employees are rarely targeted despite Washington's strong support of Calderon.
San Luis Potosi is home to a federal police academy and has not experienced many drug war killings, but gangs have been moving in to use it as a base for trafficking operations to the north.
Monterrey, Guadalajara and other Mexican cities once far from the front lines of the drug war have seen a recent spike in killings.
ICE said the two men were the first of its agents shot in the line of duty in Mexico.
If there is any evidence that drug gangs targeted the two agents, it would mark an escalation in the conflict.
“What we would hope is that there would be an incredibly strong response from the US government ... Otherwise we could have a situation where it's open season on US federal agents at the border,” said Steven Camarota of the Centre for Immigration Studies in Washington.
Enrique Camarena, an undercover US Drug Enforcement Administration agent, was kidnapped, tortured and murdered while on assignment in Mexico in 1985.
More recently, two US citizens and a Mexican linked to staff at the US consulate in Ciudad Juarez were killed in March last year, prompting the State Department to tighten security at its diplomatic missions in northern Mexico. - Reuters