Chilling photos of the Ohio gunman with his younger slain sister have emerged after the shooting rampage that left at least nine dead.

Police say 24-year-old Connor Betts opened fire in a crowded bar about 1am on Sunday, killing his sister Megan, 22, along with eight other people.

The victims were killed "in less than one minute", according to Police Chief Richard Biehl, using at least 100 rounds of bullets and a gun he bought legally, a bulletproof vest, mask and hearing protection, reports news.com.au.

Armed officers fatally shot Betts within 30 seconds of the start of his rampage, with reports stating he appeared to be targeting his sister and her boyfriend in the city of Dayton.

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Connor Betts with his sister Megan, who was among the dead in Ohio after police say he opened fire in a busy street lined with bars and restaurants. Photo / Facebook
Connor Betts with his sister Megan, who was among the dead in Ohio after police say he opened fire in a busy street lined with bars and restaurants. Photo / Facebook

In photos, taken from Megan's Facebook, the siblings appear to be close. Betts holds an arm around Megan in one image posted by their mother Moira Cofer Betts shows them a picturesque waterside location in Wilmington, North Carolina in July 2017.

"This may be my most favourite picture ever!" she captioned the photo, along with a love heart emoji.

Comments on the family photo called the siblings "cute" — something Moira responds to with "they have their moments".

A second photo shared by their mother shows them leaning in closely for a photo in a restaurant, the Crowne Plaza Dayton, dated April 19, 2017.

"My loves. So proud to be their mom!" the caption reads, along with another heart emoji.

"My loves. So proud to be their mom!" Photo / Moira Cofer Betts

Disturbing video footage shared by police this morning shows people running for their lives from the scene as rapid gunshots ring out.

On Facebook, tributes for Megan — a tour guide at Missoula Smokejumper visitor Center — began pouring in from friends for the "wonderful" young woman.

"These pictures don't even begin to show our lives together.... I have zero words. I'm so sorry this happened Megan and I love you to the moon and back," friend Libby wrote. "At least you're with God now.... Moira Cofer Betts I'm here for you and if you and Mr Betts need anything let me know... I'm absolutely sick..."

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"My heart is so broken," another said, while one wrote: "Megan was such a wonderful young lady."

Tributes to Megan have begun pouring in on Facebook from devastated friends. Photo / Facebook
Tributes to Megan have begun pouring in on Facebook from devastated friends. Photo / Facebook

Others tagged Megan's mum Moira Cofer Betts, sending their love and well wishes during the "terrible tragedy".

A message left for Moira read: "I'm so terribly sorry for this tragedy."

On Sunday night local time, a vigil is being held in the Oregon District at 8pm to honour the victims, who range in age from 25 to 57.

The popular night-life area where a gunman killed nine people and injured dozens of others has reopened to the public.

The Oregon district was reopened on Sunday afternoon, hours after the second mass shooting in the US within 24 hours.

A motive has not been released, with Police Chief Richard Biehl saying he can't yet answer why Betts began shooting early on Sunday in Dayton's Oregon District.

Officials say 27 more people were wounded, either by gunfire or as they fled. It was the second US mass shooting in less than 24 hours and came just hours after the El Paso shooting that left 20 people dead and 26 injured.

Additional reporting by AP