During the 2:30 p.m. attack near the town of Santa Maria Del Rio, a second ICE special agent, Victor Avila, also assigned to the attache office in Mexico City, was shot twice in the leg. He later was transported to the U.S. and is in stable condition.

According to U.S. and Mexican authorities, the two agents were northbound on the four-lane federal toll highway in a blue Chevrolet Suburban with diplomatic plates when they stopped at what appeared to be a military checkpoint. It was then that the unknown assailants opened fire.

No motive for the shooting has been established, although it is known that Mexican drug smugglers covet the type of truck the agents were driving and have attacked and killed others to steal them. Violence has been commonplace in Mexico since a raging war between drug-smuggling cartels began in 2006, claiming 35,000 lives.

Mexican military officers said they had no checkpoints in the area. San Luis Potosi police said the agents' bullet-ridden Suburban was found off to the side of the highway. Law enforcement authorities said at least 10 assailants were involved in the shooting, some of whom were armed with high-powered weapons.

The Mexican government does not authorize U.S. law enforcement personnel to carry weapons.

White House spokesman Nick Shapiro said President Obamatelephoned Agent Zapata's parents on Wednesday to express his condolences, telling them their son served the United States admirably and his country was grateful for his selfless service.

Homeland Security Secretary Janet Napolitano and Attorney GeneralEric H. Holder Jr. established an FBI-led task force on Wednesday to use the investigative capabilities of Homeland Security and Justice agencies to work with Mexico in tracking down the assailants.

"This joint task force reflects our commitment to bring the investigatory and prosecutorial power of the U.S. government to bear as we work with the Mexican government to bring these criminals to justice," Ms. Napolitano said. "Our thoughts and prayers are with the ICE agents' families and loved ones, as we are reminded of the risks and sacrifices undertaken every day by the men and women on the front lines in protecting the safety and security of the American people."

Mr. Holder said the shooting provided "a sad reminder of the dangers American law enforcement officers face every day.

"Working with our Mexican counterparts, we have already launched an aggressive investigation, and this joint task force will ensure that every available resource is used to bring the perpetrators of this terrible crime to justice," he said.

During a meeting, Ms. Napolitano and Mr. Holder underscored thegovernment's commitment to work with Mexican law enforcement in investigating the shooting and reiterated their commitment to broader support for Mexico's efforts to combat violence within its borders.

Ms. Napolitano also said she had spoken with Mexican Interior Minister Fernando Blake Mora, telling him that violence against Homeland Security personnel in Mexico "represents an attack against all those who serve our nation and put their lives at risk for our safety, and will not be tolerated by either country."

The Mexican government condemned what it called "this serious act of violence," and expressed "solidarity with the government of the United States and the families of the persons who were attacked."

A statement released by the Mexican office of the secretary of foreign affairs said federal authorities in that country, in coordination and support of state authorities, are making the "necessary investigations to clarify the events and bring the guilty parties to justice."

Agent Zapata joined ICE in 2006 and initially was assigned to the office of the deputy special agent in charge in Laredo, Texas, where he served on the Human Smuggling and Trafficking Unit as well as the Border Enforcement Security Task Force. He later was transferred to the U.S. Embassy in Mexico City.

He began his federal law enforcement career with Homeland Security as a member of the U.S. Border Patrol in Yuma, Ariz. A native of Brownsville, Texas, Agent Zapata graduated from the University of Texas at Brownsville in 2005 with a bachelor of science degree in criminal justice.