The U.S. attorney's office said Christian Daniel Castro Alvarez entered the plea Friday in federal court. Prosecutors say the teenager admitted entering the United States illegally from Mexico to rob a Border Patrol agent of government property.

Border Patrol Agent Robert Rosas was shot four times in the head the night of July 23 near Campo, east of San Diego. He was also shot once in the neck and three times in the torso.

Authorities say the defendant lured Rosas out of his car and struggled with him over a firearm.

Castro Alvarez faces a maximum penalty of life in prison when he is sentenced Feb. 19.