Nassau County police officers who witnessed the fatal shooting of a fellow officer last weekend said that the victim was wearing his badge and that they did not hear the officer who shot him identify himself or yell anything before firing, the president of the Nassau police union said Monday. “No one heard, ‘Stop! Police!’ ” James Carver, the president of the Nassau County Police Benevolent Association, said. However, someone seems to have yelled something: officers have said that a civilian at the chaotic scene — possibly a retired New York City police sergeant — was heard yelling “Gun! Gun!”...