(CBS NEWS) -- The Kennedy family is once again grappling with unimaginable loss.

Saoirse Kennedy Hill, a 22-year-old granddaughter of Senator Robert F. Kennedy, died Thursday at the family compound on Cape Cod, Massachusetts.

Police are investigating but a cause of death has not been announced.

RELATED: Robert F. Kennedy's granddaughter, Saoirse Hill, dies at 22

In an op-ed for her high school publication years ago, Saoirse described a struggle with mental illness:

"My depression took root in the beginning of my middle school years and will be with me for the rest of my life. Although I was mostly a happy child, I suffered bouts of deep sadness that felt like a heavy boulder on my chest. These bouts would come and go, but they did not outwardly affect me until I was a new sophomore at Deerfield," she wrote.

She described her decision to take a leave of absence from school to get treatment, and encouraged others to be open about their struggles.

MORE: Read the full story at CBS News