The discovery was made at Butterfield and Herrick roads near Herrick Lake in unincorporated Wheaton.
Authorities say someone was walking through the forest preserve when they found the human remains.
Forensic investigators combed through the area on Tuesday afternoon and evening until darkness fell. Sheriff's officers secured the scene for the night and will return on Wednesday morning to continue to search for more clues.
Investigators put up crime tape around a wide area of the forest preserve. The busy recreation area is a popular spot for fishing, hiking and picnics.
After the discovery was made, law enforcement officials closed the park while they brought in homicide investigators.
The news prompted fear and curiosity in the neighborhood.
"Kind of scary. Don't know what's going on," said Greg Cinert, resident.
Police declined to speculate publicly about the age or gender of the remains. Nor have they said how long they have been there. They put red evidence flags in spots where they found various pieces that might help the investigation. The spot where the remains were found is visible from busy Butterfield Road which runs alongside the park.
There are numerous missing persons investigations in the area and police have not said whether the remains could be linked to any of those cases.
ABC7's news partner the Daily Herald reported that the remains were badly decomposed and the coroner told the newspaper that they will be difficult to identify.
Investigators plan to take the remains to the coroner's office on Wednesday.