Early each morning, private ambulances charged toward a leafy Tomball subdivision where, for a hefty price to taxpayers, patients are dispatched to psychiatric "therapy" in a brown brick house where two portable potties are perched in the driveway. The ambulances are but specks among hundreds of emergency medical vehicles across Harris County making millions of dollars off the poor, the sick and the mentally vulnerable, whether they need a costly EMS lift or not. Medicare and the American public pay for the ride - dubiously and often - billed to the U.S. government as trips of medical necessity to "clinics" or "hospitals," a Houston Chronicle investigation has found. Much of it is fueled by the daily trafficking of the mentally ill and fragile to so-called therapy programs in aging strip malls, mid-rises and residential neighborhoods.