Carper said the shooter fired multiple rounds from "a very large gun, in technical terms".
The shooter wore body armor, Decker said.
Tuesday in MS, a shooting at a Walmart left two employees dead and a police officer wounded.
In a press conference, Mayor Whaley told reporters: "The gunman was wearing body armour and a had a 223 calibre high capacity magazines and extra magazines".
Earlier, Dayton police had said they were on the scene of an "active shooter incident" but gave no further details.
"I really want to - think about that minute".
As many as 3,000 people were inside the store during the busy back-to-school shopping season.
Police say the 24-year-old white man behind the shooting, killed his sister and eight others before he was fatally shot by police. The FBI is assisting with the investigation, Carper said. Carper further said that all the victims were found in the same place, and he pushed back on reports saying Mrs. Betts was gunned down in her vehicle along with her boyfriend.
Kettering Health Network spokeswoman Elizabeth Long said multiple victims from a shooting had been brought to system hospitals, but didn't have details on how many. Someone took the gun from him and he got shot and is dead. Earlier that night, she and another young woman had complimented each other on their outfits.
On Twitter, singer Lizzo called on people not to "normalise" shootings, saying that her family in Dayton were safe "but that's not the case for nine other families".
Ohio Gov. Mike DeWine said, "all the victims are from our own backyard, too, so this is just an enormous tragedy". The shooting took place outside, on the 400 block of East 5th Street.
Sources told the news outlet that he was from Bellbrook, Ohio, and police were searching his home on Sunday morning.
"I'm heartbroken", Whaley tweeted around 6 a.m. Sunday, thanking first responders for their efforts. "More information to come".
Carper told reporters the incident began at 1 a.m. local time in Dayton's Oregon District, a downtown historic neighborhood popular for its nightclubs, restaurants art galleries and shops.
Carper clarified that the crime scene was concentrated on East Fifth Street in Dayton's OR neighborhood, countering reports claiming that Megan Betts and her boyfriend were discovered shot to death in a vehicle at another location.
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