At least nine people have been shot dead outside a bar in Ohio, just hours after 20 people were killed in a shooting at a Walmart in Texas.
A police spokesperson in Ohio said at least 27 people had been taken to local hospitals following the attack in Dayton, with one in a critical condition and two in a "serious condition".
A senior law enforcement official named the suspected gunman as Connor Betts, a 24-year-old white male, US media reported.
The suspect is also dead after being shot by responding police officers.
Dayton mayor Nan Whaley said during a press conference that the unnamed gunman had been wearing body armour at the time of the attack.
He was using a .223-calibre rifle with extra "high-capacity" magazines, she added, but police managed to "neutralise" him in under a minute.
She said: "If Dayton police had not gotten to the shooter in under a minute… hundreds of people in the Oregon district could be dead today."
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Police at the news conference confirmed the gunman had been identified, and said that more details about his identity and possible motive would be released in due course.
Responding to questions over why the name of the gunman had not yet been released, assistant police chief Matt Carper said the investigation into the incident had taken precedence over publicly confirming a name.
Ms Whaley later stressed that the Oregon district was "one of the safest in the region" and that such an attack was rare.
Dayton Police Department had earlier tweeted that officers were patrolling in the immediate vicinity as the attack began and were able to "put an end to it quickly".
The shooting started at 1am local time (6am in the UK) in the city's Oregon district, which is home to a number of bars and restaurants.
Pictures of the scene captured by witnesses outside Ned Peppers Bar in the neighbourhood showed a police cordon surrounding the area.
One witness, James Williams, who said he was sitting outside the bar at the time of the incident, recalled a man carrying a rifle approaching the area before opening fire.
He said on Facebook: "The active shooter he tried to go into the bar but did not make it through the door someone took the gun from him and he got shot and is dead.
"A bunch of people was taken to hospital, I don't know how many – this place is a disaster."
In a further video to Facebook, Mr Williams added: "It's awful. I've never seen anything like this."
Twitter user Hannuh also said she was at the bar at the time of the incident, writing on the social media platform that she had been left "speechless and heartbroken".
She said: "We heard gunshots while on the patio…We ran for our lives and hopped a fence trampling multiple people in the process/hid anywhere we could.
We heard gunshots while on the patio.. ran indoors only to be shoved back out because the shots were coming from inside the bar. We ran for our lives and hopped a fence trampling multiple people in the process/hid anywhere we could https://t.co/pm7Q00DfbR
— Hannuh (@HannahRayNinja) August 4, 2019
Me and @KaylamILLER06 made it out of Nedpeppers safe. Scariest thing of my life. Prayers for all of the people who didnt make it out
— Taylor Mayberry (@Tayyymayyy) August 4, 2019
"I am speechless and heartbroken and bawling my eyes out. I never expected to be in such a situation and have no words. This is the most terrible thing I've ever felt.
"There are no f****** words for this feeling. It is absolutely disgusting. My heart is in pieces for anyone inside the bar who was worsely affected by this."
Another witness, Dougie Doug, broadcast live video to Facebook of the scene.
He added: "I heard the s***… there were at least 40 or 50 shots."
They just shot up ned peppers while I was in line…. police just killed dude 5 feet in front of me… I cant go out in this city anymore
— Bocock (@D_Bocock) August 4, 2019
That was a life changing experience for me in Ned Peppers
— Anthony Freeman (@AntBankS___) August 4, 2019
On Twitter, one person said he was "the next person to get my ID checked" in the line for Ned Peppers Bar, "when dude started shooting five feet from me".
In an earlier tweet, he said: "Police just killed dude five feet in front of me… I can't go out in this city anymore."
Nikita Papillon, a 23-year-old woman who was at a bar across the street when the shooting started, said she saw a girl she had previously spoken to laying outside Ned Peppers Bar.
"She had told me she liked my outfit and thought I was cute, and I told her I liked her outfit and I thought she was cute," she said.
"People my age, we don't think something like this is going to happen."