Mexican authorities have arrested two alleged leaders of the Zetas drug gang in connection with the murder of a US immigration official.
The suspected local head of the Zetas drugs gang in the northern state of Coahuila, Sergio "El Toto" Mora, was arrested by marines in Saltillo.
Federal police meanwhile detained Luis Miguel Rojo in San Luis Potosi, where the attack took place.
Another suspected gang member has been accused of killing Agent Jaime Zapata.
Julian Zapata Espinoza, who was arrested last week, told soldiers he and other gunmen opened fire on the agent's car, thinking it belonged to a rival gang, officials said.
Another US Immigrations and Customs (ICE) official, Victor Avila, was injured in the ambush outside the city of San Luis Potosi.
Mr Mora is suspected of running the Zetas' operations in San Luis Potosi.
Mr Rojo is alleged to be responsible for handling the gang's finances in the region.
Jaime Zapata, a 32-year-old Texan, was assigned to the Immigration and Customs Enforcement Agency (ICE) human smuggling and trafficking unit.
He had been on attachment to the ICE office within the US embassy in Mexico City.
Drug violence is set to be discussed when Mexican President Felipe Calderon visits President Obama this week.
More than 34,000 people have been killed in drug-related violence in Mexico since December 2006, when President Calderon's government deployed soldiers and federal police in a widespread crackdown on the illegal cartels.