SAN DIEGO (AP) — 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 the Castro Alvarez "voluntarily surrendered to the United States."

Authorities said Castro Alvarez acted with others, but he 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.

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.