The US immigration system has emerged as the nation's largest law enforcement network since the Sept. 11 attacks, but its culture of secrecy prevents the public even from knowing the names of the record 429,247 immigrants detained last year. A year-long Globe investigation reveals an insensitive, sometimes capricious system that has released thousands of potentially dangerous criminals to US streets - sometimes with deadly results. Meanwhile, thousands of immigrants with no criminal record are held for months in detention centers that treat them like criminals, even when they're sick or elderly. It's a system that the Founding Fathers never envisioned when they wrote the US Constitution.