Special Agent Jaime Zapata, on assignment to the ICE attache office in Mexico City from his post in Laredo, Texas, died when gunmen attacked the agents' blue Suburban vehicle as they drove through the northern state of San Luis Potosi.
The second agent is Victor Avila, according to US officials who weren't authorised to speak on the case. He was shot twice in the leg and transported back to the United States, where he is in stable condition, according to an ICE statement Wednesday.
President Barack Obama expressed condolences to Zapata's family.
Homeland Security Secretary Janet Napolitano said the highest-profile attack on US lawmen in Mexico since the 1985 torture and killing of DEA agent Enrique "Kiki" Camarena, won't change the US commitment to supporting Mexico in its crackdown on organized crime.
"Let me be clear: any act of violence against our ICE personnel - or any DHS 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 Tuesday. "We remain committed in our broader support for Mexico's efforts to combat violence within its borders."
US and Mexican officials said they were working closely together to investigate the shooting and find those responsible.
The two agents were driving a four-lane, federal highway from Mexico City to the northern city of Monterrey on routine business and not as part of an investigation, said a US federal law enforcement official who is not authorized to discuss the case publicly. ICE, the agency for immigration enforcement inside the US, also investigates drugs, money laundering and smuggling of weapons and other contraband in Mexico, according to former director Julie Myers.
The agents were stopped at what may have appeared to be a military checkpoint, according to one Mexican official, who was not authorized to speak publicly about the case. Mexican military officers said they had no checkpoints in the area.
After they stopped, someone opened fire on them, the official said.
It's not known whether the men were hit because they were law enforcement or because of the blue Suburban they were driving, a truck coveted by use for drug gangs. Texas missionary Nancy Davis shot to death last month in northern Mexico while travelling in a large 2008 Chevrolet pickup, and police believe the attackers were trying to steal the truck.
San Luis Potosi police said gunmen killed one person and wounded another on Highway 57 near the town of Santa Maria Del Rio at about 2:30 p.m., though they couldn't confirm they were the ICE agents. Police said a checkpoint was unlikely on such high-speed stretch of highway and that the bullet-riddled Suburban was found off to one side.
San Luis Potosi Gov. Fernando Toranzo told W Radio in Mexico Wednesday he has seen a dramatic rise in organised crime in that area as drug cartels battle for territory.
"It's had a major impact that we hadn't see before in San Luis Potosi," Toranzo said. "Right now we're waging a direct fight with all our state resources to restore order."
Mexico is fighting heavily armed drug cartels that supply the US market. Since President Felipe Calderon launched a military crackdown on organized crime shortly after taking office in December 2006, almost 35,000 people have been killed in drug-related violence.
The US has increased equipment and training support for Mexico in recent years through its $1.4 billion Merida Initiative.
As of January last year, 26 ICE special agents also had trained over 4,000 new Mexican police recruits, according to the embassy.
Zapata, who joined ICE in 2006, served on the Human Smuggling and Trafficking Unit as well as the Border Enforcement Security Task Force. He also served as a member of the US Border Patrol in Yuma, Arizona. The agency didn't provide his age but said he was a native of Brownsville, Texas, who graduated from the University of Texas at Brownsville in 2005.
Though Mexico is seeing record rates of violence, it is rare for US officials to be attacked. The US government, however, has become increasingly concerned about the safety of its employees in the country.
In March, an US employee of the American consulate in Ciudad Juarez, her husband and a Mexican tied to the consulate were killed when drug gang members fired on their cars as they left a children's party in the city across from El Paso, Texas.
The US State Department has taken several measures over the past year to protect consulate employees and their families. It has at times authorized the departure of relatives of US government employees in northern Mexican cities.
In July, it temporarily closed the consulate in Ciudad Juarez after receiving unspecified threats. Earlier this month, the consulate in Guadalajara prohibited US government officials from traveling after dark on the road to the airport because of cartel-related attacks in Mexico's second-largest city.
