A woman reacts at the site of a mass shooting where 20 people lost their lives at a Walmart in El Paso, Texas, on Sunday. Reuters

Megan Betts, 22, the sister of gunman Connor Betts, is among the fatalities in the shooting outside a bar in Dayton in the wee hours of Sunday, assistant police chief Matt Carper said.

Authorities say Connor Betts was killed by police less than a minute after he started shooting a .223-caliber rifle into the streets of a popular Dayton nightlife area around 1am on Sunday.

Mourners gather at a vigil following a nearby mass shooting in Dayton, Ohio, on Sunday. AP

They say his 22-year-old sister, Megan, was the youngest of the deceased victims, who were all killed in the same area. The others were adults ranging in age from 25 to 57.

Mayor Nan Whaley says Betts was wearing body armor and had additional high-capacity magazines. Investigators haven't publicly discussed a possible motive. It was the second US mass shooting in less than 24 hours.

President Donald Trump is ordering flags at half-staff in remembrance of the victims of two mass shootings in less than a day, which killed at least 29 people. A proclamation released by the White House on Sunday says the nation shares "in the pain and suffering of all those who were injured in these two senseless attacks."

The United States was in mourning on Sunday for victims of two mass shootings as debate raged over whether Trump's rhetoric was partly to blame for surging gun violence.

The rampages turned innocent snippets of everyday life into nightmares of bloodshed: 20 people shot dead while shopping at a crowded Walmart in El Paso, Texas on Saturday morning, and nine more outside a bar in a popular nightlife district in Dayton, Ohio just 13 hours later.

Mourner Amanda Luke holds a sign at vigil following a mass shooting in Dayton, Ohio. AP

In Texas another 26 people were wounded, and 27 in Ohio. In Dayton, the shooter, armed with a long gun, was killed by police in less than a minute. They just happened to be nearby and prevented a casualty toll that could have been many times greater, local officials said.

Still, in those few seconds the shooter managed to mow down dozens of people.

"You could see the bodies actually start to fall and we knew it was bigger than just even a shoot-out," Anthony Reynolds, who was outside the Dayton bar when the shooting started, told NBC News.

Reynolds described the shooter as a white man dressed all in black, with his face covered and armed with an assault rifle.

Police later named the gunman as a 24-year-old white man called Connor Betts and said that his sister was among those killed.

In Texas, a suspect surrendered shortly after the massacre and was described in media reports as a 21-year-old white man named Patrick Crusius who was believed to have posted online a manifesto denouncing a "Hispanic invasion" of Texas. El Paso, on the border with Mexico, is majority Latino.

Shooter 'very confident'

Prosecutors in Texas said they would seek the death penalty, and a federal official said that investigators are treating the El Paso shooting as a case of domestic terrorism.

As the country stood aghast over its latest spasm of gun violence, Trump ordered flags flown at half mast for five days.
At the Walmart, terrified shoppers cowered in aisles or ran out of the store as gunfire echoed.

Vanessa Saenz, a Walmart shopper, said the El Paso shooter was wearing a black T-shirt, cargo pants and ear protectors. She said it looked like he was "dancing."

Edie Hallberg cries while speaking to police outside a Walmart store as she looks for her missing mother. AFP

People near the shooter became cornered, and Saenz said she recalls him raising his rifle, aiming at them and shooting.
"The one thing I'll never forget is the way he walked into Walmart, very confident. He was on a mission and that's when it hit me," Saenz told ABC News Radio.

On Twitter Trump described the El Paso attack as "an act of cowardice." On Sunday morning he tweeted again saying "God bless the people of El Paso Texas. God bless the people of Dayton, Ohio."

But critics hit hard at Trump, saying his custom of speaking in derogatory terms about immigrants is pushing hatred of foreigners into the political mainstream and encouraging white supremacist thinking that encourages violence.

"To pretend that his administration and the hateful rhetoric it spreads doesn't play a role in the kind of violence that we saw yesterday in El Paso is ignorant at best and irresponsible at worst," said the Southern Poverty Law Center, a major civil rights group.

Agencies