updated 11/20/2009 5:27:42 PM ET 2009-11-20T22:27:42

A 17-year-old pleaded guilty Friday to murdering a Border Patrol agent who was shot eight times in head, neck and torso in the mountains east of San Diego.

Christian Daniel Castro Alvarez admitted entering the United States illegally to rob a Border Patrol agent, the U.S. attorney’s office said. Alvarez said he lured Agent Robert Rosas out of his car on the night of July 23 and struggled with him over a firearm.

Rosas, 30, was shot four times in the head, once in the neck and three times in the torso. He was found near Campo, Calif., without a pulse. His Border Patrol-issued gun was missing.

Rosas had left his car with the ignition on while pursuing three suspected illegal immigrants on a cool, “extremely dark” night, according to the San Diego County medical examiner’s office.

Castro Alvarez entered his plea before U.S. Magistrate Judge James Lorenz after agreeing to be tried as an adult. The U.S. attorney’s office in San Diego did not offer details on the arrest but said Castro Alvarez “voluntarily surrendered to the United States.”

Homeland Security Janet Napolitano thanked the Mexican government for cooperating.

“Today’s guilty plea makes it clear that those who harm our brave men and women in uniform will be swiftly brought to justice and punished,” she said. “I am confident that any others involved in Agent Rosas’ tragic death will also be held responsible.”

Authorities said Castro Alvarez acted with others, but did not immediately say how many.

“Our work on this case is not yet done,” U.S. Attorney Karen Hewitt said.

Castro Alvarez faces a maximum of life in prison when he is sentenced Feb. 19. His attorney, Ezekiel Cortez, did not immediately respond to a phone message.

Rosas was the first Border Patrol agent killed by gunfire since 1998, according to The Officer Down Memorial Page, a Web site that tracks deaths of law enforcement officers. His death sparked a massive search by authorities on both sides of the border in a mountainous area strewn with white boulders that is popular with illegal immigrants and marijuana smugglers.

The FBI and Border Patrol offered rewards totaling $350,000 for information leading to the arrest and conviction of those responsible, and the FBI offered an additional $10,000 for help finding Rosas’ .40-caliber gun.

Since the attack, the Border Patrol has expanded its presence in the area and been assigning agents to work in pairs instead of alone.

Rosas was patrolling alone when he volunteered to pursue a group of suspected illegal immigrants who were spotted by another agent with a night-vision scope mounted on the back of a pickup truck, authorities say. The suspects are believed to have fled back to Mexico through a small crevice under a border fence made of corrugated metal.

Just hours after the killing, Mexican authorities arrested Ernesto Parra Valenzuela, 36, near Tecate, Mexico, and identified him as the chief suspect.

Mexican officials had no immediate response on the guilty plea, but one who was not authorized to discuss the case confirmed that Castro Alvarez is a Mexican citizen.

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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