A US Immigration and Customs Enforcement agent has been killed and another wounded in Mexico.
Special agent Jaime Zapata, on assignment to the ICE Attache in Mexico City from his post in Laredo, Texas, died when gunmen attacked the agents' vehicle as they drove through the northern state of San Luis Potosi.
The second agent, who was not identified, was shot in the arm and leg and was in stable condition, according to statements from the Department of Homeland Security.
Homeland security secretary Janet Napolitano said the fatal attack on an American law enforcement agent - the highest-profile since the 1985 torture and killing of Drug Enforcement Administration agent Enrique "Kiki" Camarena - will not change the US commitment to supporting Mexico in its crackdown on organised 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," Ms Napolitano said.
"We remain committed in our broader support for Mexico's efforts to combat violence within its borders."
US and Mexican officials said they are working closely together to investigate the shooting and find those responsible.
The two agents were driving along a four-lane federal highway between Mexico City and the northern city of Monterrey when they were stopped at what may have appeared to be a military checkpoint, according to one Mexican official. Military officers said they had no checkpoints in the area.
After they stopped, someone opened fire on them, the official said.
Police said a checkpoint was unlikely on such high-speed stretch of highway and that the bullet-riddled Chevy Suburban was found off to one side.