The dogs were found in trash bags in a ditch in wooded area in a residential neighborhood of Crete Township.
According to officials they exhibited bite marks, which makes them believe they were being used for dog fighting.
The discovery was actually made several weeks ago, on March 3, by highway maintenance workers.
But it wasn't until they returned to their headquarters that the bags were opened and they realized what was inside them: six dogs in all, all of them apparently pit bulls or pit bull mixes.
There are several homes in the vicinity, but investigators believe the dogs were just ditched there and that the residents are not involved.
"They were younger dogs, should have been healthy and in the prime of life," Will County State's Attorney James Glasgow said. "They showed signs of what could be dog fighting and apparently, they had outlived their usefulness."
The state's attorney would not say how the dogs were killed.
Officials are asking for the public's help to identify the guilty parties. A $5,000 reward is being offered to anyone who provides information that leads to an arrest.