After decades of attempting to dam the flow of Mexican immigrants crossing into the United States illegally, federal agents say a new crisis is emerging along the southern border and they are helpless to stop it. Non-Mexicans are spilling over the border in record numbers - some from countries with terrorist ties - and most are set free soon after being captured. Already this year, the number of non-Mexican apprehensions has far outpaced last year's total in just eight months. And while they are still a relatively small percentage compared with the number of illegal Mexicans, critics say the federal...