Just after 6:20 a.m., Glen Ellyn police said Lake Ellyn Park was closed and asked residents to avoid the area.
There will be an increased police presence, but there is no active safety threat, police said.
The Glen Ellyn Park District Freedom 4 Run was canceled.
Police issued an update just after 9:45 a.m., saying the park had reopened.
Police did not immediately provide further information about the investigation.