Douglas County Sheriff Harvey Gjesdal said the cause of death "was not immediately apparent."
Medics and deputies responded to the scene, in the 10 block of Delaware Avenue, at about 12:20 a.m. Saturday after receiving a report of an unconscious person. Rescue personnel initiated CPR, but the victim could not be revived. He was pronounced dead at about 1:15 a.m.
The victim was later identified as Jonathan L. Gildersleeve, 18, of Rock Island.
Additional detectives were dispatched to the scene to investigate after his death, and an autopsy is planned.