Jesus Navarro Montes, a Mexican national, was arrested three days later in northern Mexico in connection with the killing and had been held over for trial there on migrant smuggling charges. The circumstances of his recent release from a Mexicali prison could not be determined Wednesday.
"We are working with a determined Mexican government and our Department of Justice to seek swift justice for the Aguilar murder," Chertoff said in a statement. "We have also assured Agent Aguilar's family that every resource is being called upon in the relentless pursuit of justice."
T.J. Bonner, president of the National Border Patrol Council, the union representing border agents, expressed disbelief when told of Montes' release. "How can a guy who murdered one of our law enforcement agents just be released like that?"
Bonner said it was his understanding that Montes was released without any restrictions. "He could be anywhere by now," he said.
Authorities believe Montes left Mexicali in Baja California in a Hummer carrying drugs and headed across sand dunes into the U.S., according to Mexico's federal attorney general's office and Public Safety Department.
Border Patrol agents saw the vehicles on Interstate 8 and pursued them. After Aguilar was struck, Montes continued across the border into Mexico and drove to Mexicali, where he gave the Hummer to accomplices for safekeeping, according to the attorney general's office.
Chertoff has cited the border agent's killing as an example of rising border violence, which he attributes to the increased desperation of smugglers and drug traffickers trying to get their goods into the U.S. Chertoff had previously praised the Mexican government's cooperation in tracking down Aguilar's killer.
In Washington on Wednesday, Mexican government spokesman Ricardo Alday said, "The United States, to this date, has presented neither a provisional order of arrest for Mr. Navarro Montes nor a formal extradition request."