An off-duty St. Louis police officer shot and killed a suspect who fired on him Wednesday, police said, and the slaying touched off unruly protests at the scene just miles from the suburb of Ferguson. The officer, 32, was working for a security company but wearing his city police uniform when he attempted to stop a man who tried to get away on foot, police said. The man, described as black and between age 18 and 20, turned and fired at the pursuing officer, who returned fire and fatally wounded him, said Schron Jackson, a spokeswoman with the St. Louis Metropolitan Police Department. Police did not disclose the names of the officer or the man who was shot. A crowd of about 200 protesters assembled at the scene in south St. Louis, a few miles from Ferguson, which has been racked by nearly nightly protests since the fatal Aug. 9 shooting of 18-year-old Michael Brown by police officer Darren Wilson.