Gwen Casten was found dead in her room in June. At the time, her family said it appeared she had died in her sleep.
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Casten's family released a statement Friday that Gwen Casten died of sudden cardiac arrhythmia.
"In layman's terms, she was fine, and then her heart stopped," the statement said.
The 17-year-old had recently graduated from Downers Grove North High School and planned to attend the University of Vermont.
In a statement, the Casten family said in part, "None of us know when our last heartbeat will come. The best we can hope for is that when our loved ones do pass, we will have no regrets about the time we were lucky enough to share. So hug the folks you love a little harder today. Be present in their lives. And spread your love a little farther. Because in the end, it's all that remains."
Note: The video in the player above is from a previous report.