DEVELOPING: Dallas police headquarters were placed on high alert Saturday due to an anonymous threat, two days after a sniper ambushed and shot 12 officers, killing five of them. Dallas police confirmed in a written statement that they had "received an anonymous threat against law enforcement across the city and has taken precautionary measures to heightened security." Maj. Thomas Castro told the Associated Press several general threats have been made against Dallas police, though nothing specific. The Dallas Morning News, citing a police source, reported that a lockdown was in effect shortly before 6 p.m. local time. There were...