The masked gunman who massacred his sister and eight others in Ohio on Sunday killed all nine in less than 30 seconds, before he was shot dead by police.

Police have released CCTV video of Connor Betts, 24, opening fire on visitors to Dayton’s popular Oregon District about 1am on Sunday (local time).

Wearing body armour and firing a .223 calibre rifle, he killed nine people and wounded dozens in just seconds. His 22-year-old sister Megan was reportedly one of the first killed, and the youngest.

Video from outside Ned Peppers Bar showed patrons fleeing for their lives. Many ran into the tavern to try to escape the hail of bullets, Police Chief Richard Biehl said.

Police officers on a routine patrol nearby arrived on the scene less than a minute after Betts opened fire.

“Had this individual made it through the doorway of Ned Peppers with that level of weaponry, there would have been catastrophic injuries and a loss of life,” Chief Biehl said.

Dayton Mayor Nan Whaley said the shooter had additional high-capacity magazines and had police not responded so quickly, “hundreds of people in the Oregon District could be dead today”.

Six of the nine killed were black, police said.

Although the possibility of a hate crime will be investigated, police said the quick timing of the violence made any discrimination in the shooting seem unlikely.

High school classmates of Betts said on Monday he had been suspended from school for compiling a “hit list” of those he wanted to kill and a “rape list” of girls he wanted to sexually assault.

“There was a kill list and a rape list, and my name was on the rape list,” the female classmate said.

Betts, who was 17 at the time, was not named publicly by authorities as the author of the list, but the former classmates said it was common knowledge he was the one suspended over the incident.

Chief Biehl said Betts and his sister had arrived in Oregon District in the same vehicle earlier on Sunday night, along with a third person. They had separated before the rampage.

Police say it is not yet clear if Betts intentionally shot his sister. The third person was among Sunday’s wounded.

Police have identified the other dead as Monica Brickhouse, 39; Nicholas Cumer, 25; Derrick Fudge, 57; Thomas McNichols, 25; Lois Oglesby, 27; Saeed Saleh, 38; Logan Turner, 30; and Beatrice Warren-Curtis, 36.

Ms Whaley said at least 27 more people were treated for injuries. At least 15 of those have been released, while several more remain in serious or critical conditions.

Betts was from Bellbrook, south-east of Dayton, and had no police record of violence.

Sunday’s shooting in Ohio came hours after a man opened fire in a crowded El Paso, Texas, shopping area, leaving 20 dead and more than two dozen injured.

Workers and patrons in front of the Dayton bar where nine people died on Sunday. Photo: Getty

Following the dual shootings, US Democratic presidential candidates have sought to blame President Donald Trump, saying his language promotes racial division and violence.

Several candidates pointed to a need for more gun restrictions, but they directed much of their criticism at Mr Trump, drawing a link between months of rhetoric against immigrants and people of colour and the shootings in Dayton and El Paso.

South Bend Mayor Pete Buttigieg said confronting white nationalist terrorism would be embarrassing for a president who “helped stoke many of these feelings in this country to begin with.”

“At best, he’s condoning and encouraging white nationalism,” Buttigieg said.

But Mr Trump has ignored gun laws and any links to racist ideology, instead connecting race and mental illness to the two mass shootings.

He sent out tweets of support after the shootings but did not appear in public until he was preparing to fly back to Washington from his New Jersey golf course, late on Sunday.

“Hate has no place in our country, and we’re going to take care of it,” Trump declared before boarding Air Force One.

-with AAP