NEW BEDFORD, Mass. -- One frigid March morning last year, federal agents raided a factory in this old whaling town, arresting hundreds of illegal immigrants as they sewed vests and backpacks for U.S. soldiers in Iraq and Afghanistan. Most were shackled and sent to a detention center in Texas, where they faced rapid deportation unless they could post thousands of dollars in bail -- money they didn't have -- to buy time to mount a defense. Then, a mystery benefactor appeared. The anonymous donor ponied up more than $200,000 to spring 40 people from detention. The payments, which until now...