Gwen Casten was found dead in her room Monday.
Casten's family said it appears she died in her sleep, but an exact cause has not been determined.
The 17-year-old recently graduated from Downers Grove North High School and planned to attend the University of Vermont.
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In a statement, the Casten family said in part, "The only thing we know about her death is that it was peaceful. And the only lesson we can take from that is to savor the moments you have with your loved ones."
Casten, D-Downers Grove, is currently running for a third term against Congresswoman Marie Newman. The Casten campaign temporarily pulled its TV ads. Newman's campaign is stopping all ads comparing the two candidates.
Newman said in a statement, "My heart breaks for the Casten family for the devastating loss of their daughter. My prayers are with Sean, Kara, and the entire Casten family."