A gunman has opened fire in the heart of Cincinnati in an attack that left him and three other people dead, police say.
The shooting sent people scrambling across the city's Fountain Square amid cries of "shooter!"
It happened at a 30-storey building, home to the corporate headquarters for regional banker Fifth Third Bancorp and other businesses, including popular ice cream, pastry and sandwich shops.
The bank building was locked down for most of the morning, and the usually crowded surrounding streets and sidewalks were closed off.
Police Chief Eliot Isaac said the shooter opened fire at about 9.10am local time at the loading dock of the Fifth Third Bank building. Isaac said the gunman then entered the bank's lobby, where he exchanged gunfire with police.
Emergency personnel and police work the scene of shooting near Fountain Square, Thursday, Sept. 6, 2018, in downtown Cincinnati. Source: AAP
It was unclear if the gunman shot himself or was shot by officers.
Hamilton County Prosecutor Joe Deters said the gunman was carrying a large amount of ammunition, and the rapid police response likely prevented many more casualties.
He said one investigator said it could have been "a bloodbath beyond imagination."
The gunman wasn't immediately identified, and police didn't comment on a possible motive. Police Lt. Steve Saunders said he wasn't a current or past employee of Fifth Third, and police didn't know immediately why he went to that building.
Police swarmed an apartment in North Bend, Ohio, a village some 25km west of Cincinnati. Deters confirmed it was the suspect's residence.
Emergency personnel and police respond to reports of an active shooter situation near Fountain Square, Thursday, Sept. 6, 2018, in downtown Cincinnati. Source: AAP
"A very horrific situation," Isaac said at the shooting scene. "We're in the very early stages" of the investigation.
Federal and state agents were on the scene as police searched the building.
Cincinnati Mayor John Cranley said the gunman was "actively shooting innocent victims."
The Fountain Square often hosts concerts, dancing, food trucks and other events around lunchtime or in the evenings and is neighboured by a hotel, restaurants and retail shops.
"It could have been any one of us," Cranley said.
He praised police who ended the threat and the response of other emergency personnel.
"It could have been much, much worse," Cranley said.
Fifth Third operates some 1200 banking centres in 10 states. The company in a statement said it's working with law enforcement and offers thoughts and prayers for "everyone caught up in this terrible event".
