The Los Angeles County Fire Department said everybody who was inside the building on Rosecrans Avenue at about 10 a.m. was evacuated.
Eleven people sustained injuries and seven were transported to a hospital. All injuries were minor except for one woman, who sustained moderate injuries due to a pre-existing condition.
The Gardena Police Department shared photos on Facebook that show the extensive damage from inside the casino.
Debris fell on some gaming tables near the lobby of the casino.
Fire officials said it all could have been much worse if the collapse would have happened in a more crowded area of the casino.
It's unknown what caused it to cave in. Fire officials also said the building had a significant water leak.
The casino remained closed Monday night and architects are on scene to work with the city to try to figure out what caused the collapse. It's unclear when the casino will be safe to reopen.