With his journey to the U.S. and back, Mr. Carrillo is helping to write the latest chapter in the American immigrant story. After years of growth, illegal immigration to the U.S. from Mexico and Central America has slowed sharply. At the same time, say demographers and immigrant advocates, more Latin American immigrants like Mr. Carrillo are apparently returning home. The impact of this shifting migration pattern is felt in the U.S. and beyond, in towns like San Juan Alotenango that depend to some degree on cash sent home by those working in the U.S.