NEW : The dead are believed to be Central and South American migrants crossing Mexico . A Mexican official said the bodies were found in a truck that was heading north from Ciudad Juarez, where they had been picked up by another group of migrants. Calderon says criminals are extorting migrants as Mexican government weakens cartels . Three suspects and a member of the Mexican military were killed in a shootout .
NEW : The dead are believed to be Central and South American migrants crossing Mexico . Brazil says at least four Brazilians were among the dead . Calderon says criminals are extorting migrants as Mexican government weakens cartels . Three suspects and a member of the Mexican military were killed in a shootout .