A U.S. Border Patrol agent won't be tried a third time for the shooting death of an illegal immigrant in the southern Arizona desert last year, prosecutors announced Thursday.

The shooting had been condemned by Mexico's president and the decision to drop the case is being criticized by human-rights activists.

Two juries this year were unable to reach a verdict in the case against agent Nicholas Corbett. Cochise County County Attorney Ed Rheinheimer's office will move to dismiss charges but could refile them if more evidence emerges, prosecutor Doyle Johnstun said.

The 40-year-old Corbett was tried on second-degree murder, manslaughter and negligent homicide charges in the January 2007 death of Francisco Javier Dominguez Rivera of Puebla, Mexico.

Corbett fatally shot Dominguez, 22, while trying to take him, two brothers and the girlfriend of one brother into custody just north of the Mexican border near Naco, hours after they had entered the country illegally.

The three witnesses testified that the 6-foot-4, 240-pound Corbett shot Dominguez without provocation as the much smaller man was in the process of kneeling to surrender.

But Corbett's lawyers argued during both trials that he shot in self-defense after Dominguez attacked him with a rock.

"Two times is generally the limit on murder trials, and we've left the door open here if we can enhance evidence in the future," said former Arizona Attorney General Grant Woods, who served as special prosecutor for Rheinheimer's office in the case.

Woods said he's interested in particular whether future technology will enable enhancement of a grainy video of the shooting incident.

Dominguez's family filed a wrongful death lawsuit Tuesday against Corbett in federal court.

Corbett has been assigned to administrative duties since being charged in the shooting. Sean Chapman, one of his attorneys, said the defense is "glad that this burden has been lifted off his shoulders."

Oscar de la Torre, Mexican consul in Douglas, said he had no immediate comment on the decision.

"It's a legal process, and we don't have comments about that," he said.

After the shooting, Mexican President Felipe Calderon condemned the incident and demanded an exhaustive investigation.

Pima County Legal Defender Isabel Garcia, a founder of the human rights group Derechos Humanos, said she wasn't surprised by the decision to dismiss the charge.

"This climate of impunity for Border Patrol agents and the attitude of total misinformation and lies about immigrants prevent, I think, any real justice," she said.