A gunman in body armour who opened fire in a popular entertainment district in Dayton, Ohio, killed nine people including his sister, officials have said.
Connor Betts, 24, was shot dead by police less than a minute after he opened fire with a .223-calibre rifle in the streets of the Oregon District at around 1am on Sunday in the second US mass shooting in less than 24 hours.
Police have not released further information about Betts or publicly discussed a motive.
The official who identified the gunman spoke on condition of anonymity.
His 22-year-old sister Megan was the youngest of those killed. The other men and women who were killed ranged in age from 25 to 57.
This was the 251st mass shooting in the US this year.
Mayor Nan Whaley said at a press conference on Sunday morning, if the police had not responded so quickly, “hundreds of people in the Oregon District could be dead today”.
"In less than one minute, Dayton first responders neutralised the shooter," Ms Whaley said.
"While this is a terribly sad day for our city, I am amazed by the quick response by Dayton Police that saved literally hundreds of lives."
The incident comes just hours after a mass shooting in El Paso, Texas, which left 20 people dead and more than two dozen injured.
Ms Whaley said the gunman, who wore body armour, was carrying a .223-calibre rifle and had additional high-capacity magazines with him.
At least 26 people have been taken to local hospitals, but the condition of the injured is unknown.
Miami Valley Hospital spokeswoman Terrea Little said 16 victims have been received at the hospital but could not confirm their conditions.
Kettering Health Network spokeswoman Elizabeth Long said multiple victims from a shooting had been brought to system hospitals but did not have details on how many.
Dayton's deputy police chief Matt Carper said it appears the suspected shooter used a long gun and fired multiple rounds.
"Fortunately we had multiple officers in the immediate vicinity when this incident started," he told reporters.
"For that we are very fortunate."
He added: "This is a very tragic incident and we are doing everything we can, to investigate it and try to identify the motivation behind this."
Police are interviewing witnesses to see if anyone else may be involved, but they believe there was only one gunman.
The FBI is assisting with the investigation of the shooting.
Officers have advised people to avoid the area and said in a tweet: "This is a large scene and investigation."
A family assistance centre is up and running at the Dayton Convention Center for family and friends looking for victims.
Local media and eyewitness reports suggest the shooting took place at or near the Ned Peppers Bar, in the heart of Oregon District - a police cordon could be seen outside the bar.
Lt Col Carper described the district as “a safe part of downtown”, home to many bars, restaurants and cinemas.
“This is extremely unusual, obviously, for any community, let alone Dayton,” Lt Col Carper said.
“In our Oregon District, this is unheard of.”
Taylor Mayberry, who said she was inside Ned Peppers when the shooting took place early on Sunday, told NBC News she hid in the bar's bathroom as “tons of gunshots” were going off.
“I just heard a few gunshots and then saw everyone jumping to the ground,” she said. “I've never experienced anything like that, so I dove to the ground and started running to the bathroom. I saw people trying to move toward the exit.”
She said she was one of the last people to be let into the bathroom, where ten other people were taking shelter.
“We were holding the door shut and waiting until the gunshots stopped,” Mayberry said.
“People were banging on the doors throughout. We weren’t sure if they were…you know…we weren’t sure if it was people getting in to shoot us or to get safe.”
The bar posted on its Instagram page writing: "All of our staff is safe and our hearts go out to everyone involved as we gather information."
Nikita Papillon, 23, was across the street at Newcom’s Tavern when the shooting started.
She said she saw a girl she had talked to earlier lying 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,” Ms Papillon said.
She had been to Ned Peppers the night before, describing it as the kind of place “where you don’t have to worry about someone shooting up the place.”
“People my age, we don’t think something like this is going to happen,” she said.
“And when it happens, words can’t describe it.”
President Donald Trump has been briefed on the shooting and praised law enforcement’s speedy response in a tweet on Sunday morning.
Governor Mike DeWine issued his own statement before 7am, announcing he has ordered flags in Ohio remain at half-mast,.
He also offered assistance to Ms Whaley.