The Congressman from Illinois posted a touching tribute to his late daughter Wednesday, revealing the moments leading up to her tragic and unexpected death.

Rep. Casten says Gwen went out for a few hours with some friends Sunday night after having dinner with the family, returned home, said goodnight to her parents, texted a friend and didn't wake up the following morning.

Sean says the only thing the family knows about Gwen's death is that it was "peaceful." Casten's office announced Tuesday Gwen had died, but details at the time were scarce.

Gwen had recently graduated from high school and was getting ready to enroll at the University of Vermont, where Rep. Casten says she was going to study environmental science.

Rep. Casten says Gwen's sudden death offers a powerful lesson -- savor every moment with loved ones.