Deputy Chief Eric Dalgleish of the Tulsa Police Department verified the death toll and said that the shooter was also dead, reportedly from a self-inflicted gunshot wound. The cause of the mass shooting was unknown. According to Dalgleish, the unnamed gunman used both a handgun and a rifle during the incident.

Dalgleish said police arrived three minutes after dispatchers received the call and made contact with the gunman one minute later. Multiple people were injured, according to police Capt. Richard Meulenberg, and the medical complex was a "catastrophic scene". The dead have yet to be identified, according to police and hospital officials.

“Officers are currently going through every room in the building checking for additional threats," police said in a Facebook post just before 6 p.m. “We know there are multiple injuries, and potentially multiple casualties."

St. Francis Health System locked down its campus Wednesday afternoon because of the situation at the Natalie Medical Building. The Natalie building houses an outpatient surgery centre and a breast health centre.

Tulsa resident Nicholas O’Brien, whose mother was in a nearby building when the shooting occurred, told reporters that he rushed to the scene.

“They were rushing people out. I don’t know if some of them were injured or just have been injured during the shooting, but some of them couldn’t walk very well. But they were just kind of wobbling and stumbling and getting them out of there," he said.

"I was pretty anxious. So once I got here and then I heard that she (his mother) was OK, the shooter had been shot and was down, I felt a lot better. It still is horrible what happened," O'Brien said.

The mass shooting in the US on June 2 comes just eight days after an 18-year-old gunman opened fire at Robb Elementary School in Uvalde, Texas, killing 19 children and two teachers before being fatally shot himself, and less than two weeks after a white man is accused of killing ten Black people in a racist attack at a Buffalo supermarket.The recent Memorial Day weekend saw multiple mass shootings nationwide, even as single-death incidents accounted for most gun fatalities.

Agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives were also at the scene, a spokesperson said. A reunification centre for families to find their loved ones was set up at a nearby high school.

(With agency inputs)