OHIO, United States — A gunman dressed in body armor opened fire in downtown Dayton, Ohio, early on Sunday, killing nine people including his sister and wounding 27 others, authorities said, in the second deadly US mass shooting in less than a day.

Police officers who were on routine patrol nearby arrived on the scene in less than a minute and shot the attacker dead, likely preventing a much higher casualty toll, police and the city's mayor said.

"In less than one minute, Dayton first responders neutralized the shooter," Mayor Nan Whaley said at a news conference. "I'm just still completely amazed at the heroic nature of our police department."

Assistant Police Chief Matt Carper named the gunman as Connor Betts, a 24-year-old white male from Bellbrook, Ohio, and said his sister Megan Betts, 22, was among those killed.

Carper told reporters the shooting began at 1 a.m. local time in Dayton's Oregon District, an historic neighborhood popular for its nightclubs, restaurants, art galleries and shops.

The motive behind the shooting was not immediately clear, and investigators believe the individual had acted alone, Carper said.

The victims were four women and five men and ranged in age from 22 to 57, authorities said, adding that the youngest was the gunman's sister. Six of the nine people killed were African-American.

"There isn't much discrimination in the shooting," Carper told reporters. "It happened in a very short period of time."

Of the 27 people injured, four remained in serious condition and one person in critical, medical authorities said.

Whaley said the suspect wore body armor and fired a rifle with .223-caliber rounds from high-capacity ammunition magazines.

Had police officers not confronted the suspect as quickly as they did, "hundreds of people in the Oregon District could be dead today," the mayor said.

FBI agents were assisting in the investigation.

The shooting in Dayton, a riverfront city of about 140,000 people in southwestern Ohio, came just 13 hours after a mass shooting at a Walmart store in El Paso, Texas, where 20 people were killed and 26 others wounded. The 21-year-old suspect in that shooting was arrested.

The Ohio shooting was the third major outbreak of US gun violence, coming seven days after a teenager killed three people with an assault rifle at a food festival in Northern California before taking his own life.

The latest shooting occurred outside a Dayton tavern called Ned Peppers Bar.

The Dayton Daily News cited a Facebook post from James Wilson, who said he was a customer sitting on a patio just outside the bar when the shooting occurred in front of the establishment.

"He (a gunman) tried to get into the bar but did not make it through the door," Wilson wrote. "Someone took the gun from him and he got shot and is dead."

One witness, Anthony Reynolds, said he heard gunfire that sounded like it was coming from a high-powered weapon.

"Just boom boom boom boom boom boom rapid," he said. "You could tell there's a big gun. You're not going to get those from no handgun. You're not."

Deb Decker, a spokeswoman for emergency services in Montgomery County, Ohio, told CNN the assailant had been making his way to Ned Peppers from another bar when someone grabbed the barrel of his rifle, and he drew a handgun, but was then shot as police arrived.

The mayor said the carnage in Dayton marked the 250th mass shooting in the United States so far this year, a figure that could not immediately be verified. — Reuters