Green Bay police and the Brown County Sheriff's Office told the Associated Press they have no details.
Webster cited Oneida police and casino security for her information. She said people were being cleared out of the casino and nearby properties.
"We do not know where the individual was shooting, or [where] individuals were shot," Webster said.
The Oneida Casino is operated by the Oneida Nation. Webster said the casino is connected to a large hotel and conference centre, the Radisson, also owned by the Oneida Nation.
Gambler Max Westphal said he was standing outside after being evacuated for what he thought was a minor issue.
"All of a sudden we hear a massive flurry of gunshots — 20 to 30 gunshots for sure," Westphal told WBAY-TV. "We took off running towards the highway. There had to have been 50 cop cars that came by on the highway. It was honestly insane."
Governor Tony Evers said he was "devastated" to hear about the shooting, but gave no details.
"While we are waiting for more information, we hope and pray those who were injured will recover and are grateful for the first responders who quickly responded to the situation."
The Oneida tribe's reservation lies on the west side of the Green Bay area.