SAN DIEGO -- A Border Patrol agent who was killed last month in a remote, boulder-strewn area was shot eight times in the head, neck and torso, according to an autopsy report released Tuesday.

Robert Rosas, 30, was struck four times in the head -- three times on the left side of his face and once in the back of his head, the San Diego County medical examiner's office. He was also shot once in the neck and three times in the torso.

The autopsy said Rosas and other agents were pursuing three suspected illegal immigrants on a cool, "extremely dark" night in Campo, about 60 miles east of San Diego. Rosas, driving alone, left his vehicle with the ignition running. The report says Rosas approached "at least one person," but does not elaborate.

Another agent who heard six to eight gunshots rushed to Rosas, finding him without a pulse about 10 yards from his flashlight and another 10 yards from his vehicle. A wallet in his pocket contained his driver's license and badge.

Mexican authorities have identified the chief suspect as Ernesto Parra Valenzuela, 36, who was arrested near Tecate, Mexico, several hours after the July 23 killing. The FBI, which is leading the investigation in the United States, has not identified any suspects.

Late last month, a judge in Mexico ordered that Parra Valenzuela be held during the investigation. It was unknown if Parra Valenzuela had an attorney.

The FBI and Border Patrol are offering rewards totaling $350,000 for information leading to the arrest and conviction of those responsible, and the FBI is offering an additional $10,000 for help finding Rosas' Border Patrol gun.

Rosas is the first Border Patrol agent to be killed by gunfire since 1998, according to The Officer Down Memorial Page, a Web site that tracks deaths of law enforcement officers.