Investigators said the two brothers used to be co-owners, but now the restaurant is only owned by one of them.
Fox Lake police said they believe the brothers got into an argument in the vicinity of the restaurant's kitchen and shots were fired.
Police said the 56-year-old restaurant owner's brother shot him multiple times, and the victim collapsed outside as he tried to get away. That's when police said the other, 63-year-old brother took his own life.
"I was just shocked, I was totally shocked and I didn't know who it was - it just said two people in the kitchen," said Gary Klatt, who knows the restaurant owner.
The 56-year-old was critically injured and taken to the hospital for treatment.
Neighbors who know the owner are now heartbroken for him and his family. Norine Hileman said he is like a son to her.
"My best friend, he called me every single day, 'Mom, how are you? Mom, do you need anything?'" she said.
There were other people in the restaurant at the time of the shooting, police said.