The Ohio gunman who killed nine people on Sunday had previously been suspended from school over a "hit list" of classmates and a "rape list" of girls he wanted to sexually assault.

Connor Betts, 24, opened fire outside a crowded bar in the early hours of Sunday, injuring dozens and leaving his own sister among the fatalities, before police on patrol in the area shot him dead.

It has now emerged that the gunman had a history of violence, leading to questions over how he was able to purchase the fire arms used in the shooting spree.

According to high school classmates, Betts was suspended during their junior year at suburban Bellbrook High School after a hit list of those he wanted to kill was found scrawled in a school bathroom.

That followed an earlier suspension after Betts came to school with a list of female students he wanted to sexually assault, according to the two classmates, a man and a woman who are both now 24 and did not wish to be identified.

"There was a kill list and a rape list, and my name was on the rape list," the female classmate told the Associated Press.