MEXICO CITY (Reuters) - Mexico demanded a U.S. investigation on Monday into the death of an undocumented Mexican shot by a U.S. Border Patrol agent as he tried to cross into California last week. Raul Martinez, a spokesman for the U.S. Border Patrol in San Diego, confirmed that an agent had fired on a man on Friday as he tried to cross the border near San Diego, after the would-be immigrant had thrown rocks at him. The unidentified man crossed back into Mexico, where he later died of his injuries in a hospital. "The Mexican government is demanding an investigation...