Mrs. Clinton has said she gave the State Department more than 30,000 emails related to her work in office. She deleted about the same number of emails that she said were personal. Her lawyer has said the server was then wiped clean. Mrs. Clinton has also said that she had no classified information on her account.

In a statement, a spokesman for Mrs. Clinton defended her use of the account. “Hillary Clinton’s use of a personal email account was consistent with the practice of other secretaries of state, and permissible under the department’s policy at the time,” said the spokesman, Brian Fallon.

When Mrs. Clinton became secretary in February 2009, the State Department’s general policy was “that normal day-to-day operations be conducted on an authorized” government computer. Nine months later, federal regulations were toughened to say that government agencies that allow employees to use nongovernment email accounts must “ensure that federal records sent or received on such systems are preserved in the appropriate agency record-keeping system.”

Since The New York Times reported in March that Mrs. Clinton exclusively used a personal email account, she has said that she complied with the regulations by sending emails to the work accounts of government officials so that those messages would be caught in the government’s servers. At a news conference in March at the United Nations, Mrs. Clinton said, “I fully complied with every rule that I was governed by.”

Emails from Mrs. Clinton’s account that were handed over to Congress show that she sent emails to at least four of her aides on their personal email accounts. Unless those emails were later sent to government accounts, they could not have been retained on government record-keeping systems.