A Tennessee county linked to the discriminatory treatment of teenaged African-American suspects has signed a landmark agreement to reform its practices. On Monday, the Justice Department said it had reached a deal with local officials to overhaul juvenile justice in Shelby County. A report earlier this year found black teens were twice as likely to be detained than whites and were often sent to adult criminal court for minor offenses. Juvenile suspects also attempted suicide at record rates and were detained in harsh conditions, including being tied to restraint chairs and left alone for far longer than allowed by law. Under the agreement, Shelby will be forced to carry out a series of reforms to avoid future lawsuits by federal prosecutors.