Four people were killed Thursday when a gunman opened fire at an office building downtown. Police said that officers exchanged gunfire with the shooter, but it was not immediately clear if they shot him or he killed himself. City Councilman P.G. Sittenfeld tweeted that five people had been shot, including the gunman, and that police had secured the scene. Sittenfeld said the situation remained fluid.   No motive for the carnage had immediately been determined.  “Terrible shooting incident in the heart of our city this morning,” Sittenfeld tweeted. “Multiple shot, and tragically there are fatalities. Details still emerging. Pray for our city.”  A phalanx of police officers and ambulances rushed to the scene at the Fifth Third Center, a 30-story building in the heart of downtown, during the morning rush. Four victims were transported to the University of Cincinnati Medical Center, hospital officials said.