TULSA, Oklahoma - A bad mix of chemicals at the Osage Casino in Tulsa caused 13 people to seek medical attention. 

The Tulsa Fire Department and HAZMAT crews worked Sunday afternoon to figure out what went wrong. 

Firefighters said while an employee was cleaning, he accidentally mixed ammonia and bleach, a dangerous combination.

The Tulsa Fire Department said several people complaining of having a scratchy throat, burning eyes and headache caused an Osage Casino employee to call 911. 

By the time firefighters got there, they said four people had already taken themselves to either urgent care or the hospital. 

"We had EMSA set up a triage area on the south side of the building and we treated around nine people, I believe, EMSA did," said Tulsa Fire Department Captain Keith Beck. 

As HAZMAT crews worked in the back of the building, firefighters wanted to evacuate the casino.

"That was in the process of happening about the time that we figured out that the hazard had pretty much been mitigated," said Beck. 

They discovered several chemicals, including bleach and ammonia, were mixed by an employee hours earlier, before he left work. 

"It was somebody that was actually trying to clean and clean some stains off the floor and stuff. It wasn't malicious or anything at all. It was just an accident," said Beck.  

He said it's a dangerous combination. 

"I can just tell you from a safety standpoint, don't mix ammonia and bleach," he said. 

In a statement, Osage Casino CEO Byron Bighorse said, in part: 

"We can confirm that no one was injured in this morning's incident, and it occurred in an area where Casino guests were not affected. The safety of our guests and employees is of the utmost importance, and after a thorough review from the Tulsa Fire Department and EMSA, it was determined the area was safe and evacuation was not necessary."

"There's people in there gambling, having a great time, enjoying their Sunday afternoon," Beck said. 

Firefighters said they expect the four people who drove themselves to urgent care or the hospital should be just fine.