Reportedly four people, including the gunman, died.
Isaac said investigators looked at the 9mm handgun that Santa Perez bought legally in August and noticed it had jammed at some point.
Witnesses reported seeing victims on stretchers.
The chaos unfolded alongside Fountain Square, the heavily trafficked park at the city's center. He had completed his post-graduation and was working as a financial consultant in a bank in Cincinnati.
"What we found out about Mr. Perez, we have not been able to determine what his actual motive was", Isaac said.
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Cincinnati Mayor John Cranley said the gunman was "actively shooting innocent victims" and that it was a "horrific" scene.
One onlooker, Leonard Cain, told a newspaper he had been about to enter the bank when someone warned him about the shooting. He said the shooter, who fired a 9-millimeter semi-automatic pistol during the incident, was not a former or current employee of the bank.
The Hamilton County Coroner's Office has identified those victims who lost their lives as 25-year-old Pruthvi Raj Kandepi, 64-year-old Richard Newcomer, and 48-year-old Luis Felipe Calderón.
Police body camera video played Friday showed officers taking cover behind a corner of the building on some stairs and firing through the glass.
A spokeswoman for the University of Cincinnati Medical Center said three men and one woman were treated there for gunshot wounds.
"The shots got closer and closer and closer and closer and that's all you can think about is, I'm next, I'm next, I'm next", Janetta Cook who works at a bakery shop near the lobby told CNN affiliate WCPO.
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Santa Perez had been in Cincinnati since at least 2015, police said.
"I kind of got down on the ground and then all the cops started coming", he said.
"It could have been any one of us", Cranley said. "Our thoughts and prayers go to the families of the victims, our heartful gratitude goes to the first responders and our continued support goes to this community that we love".
Police earlier characterized it as an "active shooter/officer involved shooting incident" in a tweet, but did not elaborate. Another moved to Cincinnati a year ago to take a better job with Fifth Third Bancorp.
Ohio Gov. John Kasich condemned the rampage as "a senseless act of violence on the streets of Cincinnati". "That's not what our city is", Sittenfeld wrote on Twitter.
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