City officials in Dayton, Ohio, held a press conference Monday in the wake of a mass shooting that left nine dead in the country's second deadly shooting rampage over the weekend. Police said the gunman was wearing a mask and body armor when he shot and killed his younger sister and eight others after the siblings arrived together with a friend Saturday evening at a popular entertainment district packed with people.

It all happened within 30 seconds before police officers stationed nearby shot and killed 24-year-old Connor Betts, who was armed with a .223-caliber rifle, Dayton Police Chief Richard Biehl said. 

Authorities haven't yet determined a motive. While the gunman was white and six of the nine people killed were black, police said the quickness of the rampage made any discrimination in the shooting seem unlikely.

Speaking Monday, Biehl said that the shooter could have carried 250 rounds if the magazines he had were at full capacity, but it's not clear whether the magazines were full. The 250 includes loose rounds found near the suspect and in a backpack he carried, Biehl said. Investigators uncovered at least 41 spent shell casings from the shooter's weapon, Biehl said.  

Biehl said the shooter had modified the weapon, but didn't detail how he did so. 

"That level of weaponry in a civilian environment, unregulated, is problematic," Biehl said.

Biehl said there was conflicting information about the number of people injured -- either 27 or 32. 14 of the injured suffered gunshot wounds, and the others were injured fleeing the scene. 11 victims remained hospitalized Monday, a fire official said.

Biehl said it's still unknown whether Betts targeted any of the victims, including his 22-year-old sister, Megan, the youngest of the deceased victims.  

"It seems to defy believability he would shoot his own sister," Biehl said. "It's also hard to believe he didn't recognize that was his sister. So we just don't know."