In Ohio, more than 30 more people had injuries, including at least 14 who had gunshot wounds and others who suffered non-firearm injuries as people fled, city officials said.
"You can well understand in a very dynamic and chaotic scene as this was, with people running through the crime scene, with first responders running through the crime scene, with EMS traveling through the crime scene, evidence can be lost", the police chief told reporters.
Betts was armed with an AR 15-style rifle, and if all of the magazines he had with him were full, which hasn't been confirmed, he would have had a maximum of 250 rounds, Biehl said.
Betts was gunned down within 30 seconds of the start of his rampage. It all happened within 30 seconds, before police officers stationed nearby fatally shot Betts.
Had he gotten inside the bar, the result would have been "catastrophic", Biehl said.
He said that level of firepower in the hands of an unregulated civilian was "fundamentally problematic". Authorities are still investigating a motive for the massacre. He later opened fire, carrying extra ammunition in a backpack.
But he added investigators are baffled over whether Emily and her male companion were one of Betts' intended targets.
"Dayton has been through more than we could ever have imagined this year", said Dayton mayor Nan Whaley, noting that the city recently experienced a major tornado outbreak that left one dead and 12 injured.
Biehl said authorities were still trying to understand why Betts shot his sister, who had driven earlier in the evening to the OR nightlife district with Betts and another companion, who was wounded in the shooting.
"It seems to just defy believability that he would shoot his own sister", Biehl said. "Any suggestion at this time of motive would be irresponsible".
Ohio's two US senators also visited Dayton.
For all the back-to-back horror of innocent people slain amid everyday life, decades of an unmistakably American problem of gun violence ensured it wasn't entirely shocking. Biehl said the shooter arrived at the nightlife spot in the same auto as his sister Megan Betts, 22, and a friend, "but they were separated at some point later in the evening".
What do we know about the victims?
David Partridge, 26, who also attended Bellbrook with Betts, said the list included a member of his family.
Sugarcreek Township police did not return a phone seeking comment. The 250 includes loose rounds found near the suspect and in a backpack he carried, Biehl said.
"We are clearly exploring every possible piece of information", Biehl said.