Christine Blasey Ford sent an unusual Facebook message to her best college friend this summer with a question: Had she ever mentioned a sexual assault that occurred when she was in high school?

Her friend, Catherine Piwowarski, her onetime roommate at the University of North Carolina at Chapel Hill, said she had no memory of that. She didn’t know at the time that Dr. Ford was considering coming forward with her allegation that Supreme Court nominee Brett Kavanaugh had sexually assaulted her in the 1980s, when both were teenagers...