DALLAS — Snipers opened fire on police officers in the heart of Dallas, killing five officers and injuring six others during protests over two recent fatal police shootings of black men, police said. Three people are in custody and a fourth suspect was exchanging gunfire with authorities in a parking garage downtown, Dallas Police Chief David Brown said early Friday morning. The suspect is not cooperating and has told negotiators he intends to hurt more law enforcement officials, the chief said.