Nine people were killed and 27 others were wounded, the police said. It was the second American mass shooting in 24 hours, and the third in a week. The gunman who killed nine people, including his sister, early Sunday morning in Dayton, Ohio, wore a mask, body armor and hearing protection, and he possessed a high-capacity magazine capable of holding 100 rounds, the police said. The gunman, identified as Connor Betts, 24, used what the police described as an “assault-style rifle” when he opened fire in a busy entertainment district in Dayton. A shotgun was also found in Mr. Betts’s...