U.S. officials were shocked that a Mexican judge had freed a man imprisoned in connection with the death of a U.S. Border Patrol agent, Homeland Security Secretary Michael Chertoff said.

Jesus Navarro Montes was arrested Jan. 22 in northern Mexico in the killing of Agent Luis Aguilar and had also been held over for trial there on migrant smuggling charges. The circumstances of his recent release from a Mexicali prison couldn't 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 Wednesday. "We have also assured Agent Aguilar's family that every resource is being called upon in the relentless pursuit of justice."

The attorney general's office in Baja California, Mexico, confirmed that Navarro had been released but couldn't provide any details of his case. Officials answering phones at the 12th District Court in Mexicali said no one authorized to speak to the media was available.

Aguilar was run over and killed Jan. 19 as he tried to put down spike strips to stop a drug-filled vehicle and a pickup that were fleeing back to Mexico.

Authorities believe Navarro 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 in southeastern California and pursued them. When both vehicles turned off the highway toward Mexico, Aguilar tried to deflate the vehicles' tires but was struck.

Navarro 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. He was arrested a few days later.

Mexican officials announced in late January that a judge had ordered a trial for Navarro on migrant smuggling charges.

It wasn't clear Wednesday whether the U.S. had formally requested his extradition. But Mexican law usually requires that suspects face justice in Mexico before they can be extradited.

Debra Hartman, a spokeswoman for the U.S. attorney's office in San Diego, wouldn't say Wednesday whether the U.S. government had charged Navarro or had sought his extradition. She wouldn't say whether the investigation of the case had been completed.

An online federal court database does not list any charges filed by prosecutors against Navarro in Southern California.

The head of the union representing Border Patrol agents also expressed shock over Navarro's release. "Every Border Patrol agent in the country is outraged and stunned by this," said T.J. Bonner, president of the National Border Patrol Council.