Border Patrol agent Robert Rosas Jr.

A Border Patrol honor guard stands at attention before carrying agent Robert Rosas' flag-drapped coffin during a memorial service in El Centro, Calif., in 2009. (Don Bartletti / Los Angeles Times / July 31, 2009)

SAN DIEGO -- A 28-year-old Mexican national was sentenced Thursday to 56 years in federal prison for his part in the robbery and killing of a Border Patrol agent.

Marcos Rodriguez-Perez is the second co-defendant sentenced in the killing of  agent Robert Rosas Jr. near Campo in southern San Diego County on July 23, 2009.

Another co-defendant, Christian Daniel Castro-Alvarez, was sentenced in April 2010 to 40 years.

Two others, Jose Luis Ramirez-Dorantes and Emilio Samyn Gonzales-Arenazas,  have pleaded guilty and are awaiting sentencing.

A fifth suspect, Juan Chacon-Morales, is a fugitive. The FBI is offering a reward of up to $100,000 for information leading to his arrest.

The five -- all of whom, prosecutors said, were crossing into the U.S. illegally -- are accused of detaining Rosas at gunpoint, shooting him multiple times and then stealing his weapon, night-vision device and other equipment before fleeing back to Mexico.

Rodriguez-Perez was arrested by Mexican officials in Tijuana and extradited to the United States. He pleaded guilty to conspiracy to commit robbery and kidnapping, robbery of U.S. property, and use of a firearm during a crime of violence.

"If a law enforcement officer is harmed, we'll use every resource we have to catch the perpetrators and never let it be doubted: We will find them," U.S. Atty. Laura Duffy said.

U.S. District Judge M. James Lorenz ordered Rodriguez-Perez's sentence to run consecutive to a two-year sentence for violating his supervised release from a prior smuggling conviction.

Rosas, 30, had been with the Border Patrol for three years. He was survived by his wife, son and daughter.

"This sentencing is tempered with the sobering reality of the senseless loss of a fine man, husband and father," said Chief Patrol Agent Paul Beeson of the Border Patrol's San Diego sector.

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tony.perry@latimes.com