Philadelphia (AP) - In the United States, an unidentified gunman opened the fire and shot at least six policemen. The situation in Philadelphia is not yet under control and no one should approach the scene, the police warned on Twitter. "The suspect is still firing," wrote a police spokesman. Television pictures show a large-scale police operation with dozens of officers, including heavily armed special forces in protective clothing. The shooter reportedly barricaded himself in a house. A local resident told broadcaster 6ABC that she had heard at least 100 shots.