Top Senate Judiciary Democrat Dianne Feinstein notified the FBI about unspecified information she received about U.S. Supreme Court nominee Brett Kavanaugh, injecting confusion even as Republicans set a path to confirm him by the end of the month.

“I have received information from an individual concerning the nomination of Brett Kavanaugh to the Supreme Court,” Feinstein said in a statement Thursday. “That individual strongly requested confidentiality, declined to come forward or press the matter further, and I have honored...