“Secretary Clinton gave a voluntary interview this morning about her email arrangements while she was secretary,” Nick Merrill, a campaign spokesman, said in a statement. “She is pleased to have had the opportunity to assist the Department of Justice in bringing this review to a conclusion.”

The campaign declined to elaborate on the substance of the interview, which lasted about three and a half hours at F.B.I. Headquarters in Washington. Shortly afterward, two black S.U.V.s were seen returning to Mrs. Clinton’s house in the capital.