Mexican drug cartels whose operatives once rarely ventured beyond the U.S. border are dispatching some of their most trusted agents to live and work deep inside the United States — an emboldened presence that experts believe is meant to tighten their grip on the world's most lucrative narcotics market and maximize profits. If left unchecked, authorities say, the cartels' move into the American interior could render the syndicates harder than ever to dislodge and pave the way for them to expand into other criminal enterprises such as prostitution, kidnapping-and-extortion rackets and money laundering. Cartel activity in the U.S. is certainly...