Washington (CNN) Retired Justice Sandra Day O'Connor revealed in a letter on Tuesday that she has been diagnosed with the "beginning stages of dementia, probably Alzheimer's disease."

"I will continue living in Phoenix, Arizona surrounded by dear friends and family," she wrote and added, "While the final chapter of my life with dementia may be trying, nothing has diminished my gratitude and deep appreciation for the countless blessings of my life."

Chief Justice John Roberts praised O'Connor in a statement Tuesday as a "towering figure" and a "role model not only for girls and women, but for all those committed to equal justice under law."

O'Connor, 88, was nominated to the bench by President Ronald Reagan as the first female Supreme Court justice of the United States in 1981. She retired from the bench in 2006, in part to care for her husband, who was ailing from Alzheimer's.

In her retirement, she became an advocate for Alzheimer's disease as well as launching iCivics, a website dedicated to encouraging young people to learn civics.

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