Mexican police announced the arrests Saturday of [Jose Quintero Ruiz, 43, and his brother Jose Eugenio Quintero Ruiz, 49, and taxi drivers Jose Alfredo Camacho, 34 and Antonio Valladares, 57] suspected of involvement in the killing of . . . U.S. Border Patrol agent [Robert Rosas] as American investigators searched hospitals for gunmen who were possibly wounded in the first such shooting death in more than a decade. . . .
American officials have expressed concerns that the drug cartel battles plaguing Mexico could spill into the United States with the targeting of U.S. law enforcement officials. [The head of the FBI's San Diego bureau] said investigators aren't ruling out the possibility that Rosas was slain by drug smugglers or even human smugglers.