The initial incident started at the Duck Creek restaurant at the main casino located in Green Bay, near Austin Straubel Airport.
Lt. Kevin Pawoak and Chief Joel Maxam of the Brown County Sheriff's Office held a press conference a few hours after the shooting and said that some victims were shot inside the Radisson Hotel that is connected to the Oneido Casino.
"Our investigative team is actively working to obtain eyewitness statements and process evidence," the sheriff's officer said in a statement Sunday afternoon.
Officials said investigators would likely be working at the casino collecting evidence into Sunday night.
The names of the gunman and the victims are not being released until notification is made to relatives, the sheriff's department said.
Authorities confirmed that every agency within Brown County responded to the scene, including local fire departments and rescue squads, and that DCI was also assisting along with the FBI.
Pawoak and Maxam suggested that there is some possible relationship regarding employment with the shooter during the press conference but did not elaborate.
No information on the kind of weapon that was used in the shooting was given but the target was believed to be an employee. It is currently unclear if the shooter was an employee or had a personal relationship with the employee in the restaurant.
Police say the casino was immediately cleared out and that witness statements are still being taken.
The sheriff's department asked that anyone who witnessed the shooting to contact homicide detective immediately.
Gov. Tony Evers and his wife, Kathy Evers, released statement saying they were "devastated" to hear about the casino shooting.
"Our hearts, thoughts, and support go out to the Oneida Nation, the Ashwaubenon and Green Bay communities, and all those affected by this tragedy," said Evers. "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."
Wisconsin Attorney General Josh Kaul said on Twitter the situation was contained and there was no longer a threat to the community.
Officials with Oneida Nation said all casino locations would be closed until further notice.
ABC News' Bill Hutchinson contributed to this report.