Mrs. Clinton’s aides used their private email accounts to communicate with her at times.

Not only was Mrs. Clinton corresponding from a private email account, at times four of her closest advisers — chief of staff, Cheryl Mills; senior adviser, Philippe Reines; personal aide, Huma Abedin; and deputy chief of staff, Jake Sullivan — were doing so.

That raises questions about whether the exchanges would have been captured on public record searches at the State Department.