BROWNSVILLE, Texas -- Members of Mexico's ruthless Zetas gang carried out a highway ambush that killed one U.S. federal agent and wounded another this week, a Texas congressman said Thursday.

Michael McCaul said Immigration and Customs Enforcement Special Agent Jaime Zapata, 32, was killed by members of the Zeta cartel after a group of 10 to 15 armed men in two vehicles forced Zapata's Chevy Suburban off a highway in San Luis Potosi on Tuesday afternoon. ICE Agent Victor Avila was shot twice in the leg.

McCaul, chairman of the House Homeland Security Oversight and Investigations Subcommittee, said the agents identified themselves as U.S. diplomats before being shot.

"This was a complete ambush," he said, adding that investigators recovered least 90 bullet casings from the scene.

The Texas Republican said Zapata and Avila identified themselves as U.S. diplomats "hoping they (the Zetas) would honor the long-standing tradition that they don't (target) U.S. law enforcement."

"This is a complete game changer," he said. "They are changing the rules."

He said while the motive for the attack remains unclear, one thing is certain: "There's no case of mistaken identity."

Authorities have said the agents were likely in the wrong place at the wrong time and that their SUV is of a kind coveted by drug cartels in the area.

Mexican authorities are investigating the shooting but have not announced any arrests.

"My sense is that we know, we probably have pretty good intelligence as to who was behind this," McCaul said. "That's what it appears to be."

U.S. Homeland Security Secretary Janet Napolitano and Attorney General Eric Holder have formed a joint task force led by the FBI to help Mexico find the killers.

Holder said Thursday that U.S. officials would look closely at the security situation in Mexico and make any changes warranted to ensure that U.S. personnel "have maximum protection."

The Mexican government does not allow U.S. law enforcement personnel to carry weapons. Holder would not say if the government planned to push to allow U.S. law enforcement working in Mexico to arm themselves.

McCaul was more forthright.

"We are helping them, they are not helping us," McCaul said. "If we are going to put our guys down there ... to allow them not to be armed really puts them right in the bullseye and they are sitting targets."

McCaul said he wants to schedule congressional hearings to examine the U.S. role in Mexico's ongoing drug war, which has killed more than 35,000 people since Mexican President Felipe Calderon launched an offensive against the country's drug gangs shortly after taking office in late 2006.

Zapata and Avila, both from Texas, were on temporary assignments at the ICE attache office in Mexico City. They had been driving between Monterrey, Mexico, and Mexico City when the attack occurred.

Avila serves on a unit to deter human trafficking and is based in El Paso, Texas. Zapata had been based out of Laredo, Texas. He joined Homeland Security in 2006, served on the Human Smuggling and Trafficking Unit as well as the Border Enforcement Security Task Force. He also was a member of the U.S. Border Patrol in Yuma, Ariz.

On Thursday, police and ICE officers blocked the entrance to the road leading to Zapata's family home in Brownsville, Texas.

Brownsville police spokesman Eddie Garcia said the department doesn't usually provide such protection to the family of shooting victims but that it was appropriate given the circumstances.

The security is "for the privacy of the family. The family is not ready to come out yet and give interviews," Garcia said.

The U.S. State Department has taken several measures over the past year to protect consulate employees and their families in Mexico, although attacks on Americans are rare. It has at times authorized the departure of relatives of U.S. government employees in northern Mexican cities.

In July, it temporarily closed the consulate in Ciudad Juarez, a city at the center of Mexico's drug cartel violence, after receiving unspecified threats. Earlier this month, the consulate in Guadalajara prohibited U.S. government officials from traveling after dark on the road to the airport because of cartel-related attacks in Mexico's second-largest city.