An armed man opened fire in the heart of Cincinnati in an attack that left him and three other people dead, police said.
The shots sent people running through the Plaza de la Fuente in the city amid cries of "shooter".
It happened in a 30-story building, home to the corporate headquarters of regional banker Fifth Third Bancorp and other companies, including popular ice cream, pastry and sandwich shops.
The bank building was closed for most of the morning, and the streets and sidewalks that were usually crowded were closed.
Police Chief Eliot Isaac said the shooter opened fire around 9.10 a.m. local time at the loading dock of the Fifth Third Bank building. Isaac said that the gunman then entered the lobby of the bank, where he exchanged gunfire with the police.
It was not clear if the gunman shot himself or was shot by officers.
Hamilton County prosecutor Joe Deters said the gunman carried a large amount of ammunition, and that the quick response from the police probably prevented many more casualties.
He said an investigator said it could have been "a bloodbath beyond imagination."
The gunman was not immediately identified, and the police did not comment on a possible motive. Police Lieutenant Steve Saunders said he was not a current or past employee of Fifth Third, and the police did not immediately know why he went to that building.
Police invaded an apartment in North Bend, Ohio, a village about 25 miles west of Cincinnati. Deters confirmed that it was the residence of the suspect.
"A very horrible situation," Isaac said at the scene of the shooting. "We are in the early stages" of the investigation.
Federal and state agents were on the scene while the police searched the building.
The mayor of Cincinnati, John Cranley, said the gunman was "actively shooting innocent victims."
The Plaza de la Fuente often hosts concerts, dances, food trucks and other events at lunchtime or at night and is surrounded by a hotel, restaurants and retail stores.
"It could have been any of us," Cranley said.
He praised the police that ended the threat and the response of other emergency personnel.
"It could have been much, much worse," Cranley said.
Fifth Third operates around 1,200 banking centers in 10 states. The company said in a statement that it is working with the police and offers thoughts and prayers for "all those who are trapped in this terrible event."