Police in the Seattle suburb of Bellevue had a chance to get Carlos Daniel Carillo-Lopez, an illegal immigrant, off the streets in March — but a local jail defied a deportation notification request from U.S. Immigration and Customs Enforcement and released him into the community. Three weeks later, police say, the 19-year-old Guatemalan, eager to join the violent Surenos street gang, took part in a murder posse that tracked down and slew a teen from a rival gang. Mr. Carillo-Lopez, who came to the U.S. as part of the Unaccompanied Alien Child (UAC) surge, is at the crossroads of the...