The specialist then used a program known as BleachBit to delete an unknown number of emails, according to the report. Mrs. Clinton told investigators that she was unaware that the aide had deleted the emails.

Dozens of times during her interview, Mrs. Clinton said she did not remember details about the server or guidance she had received on how to handle classified information.

In its summary of the investigation, the F.B.I. said that Mrs. Clinton had emailed Colin Powell, a former secretary of state, a day after she was sworn in to office about Mr. Powell’s use of a personal email account when he was the country’s top diplomat. Mr. Powell warned Mrs. Clinton that if she used her BlackBerry for official business, those emails could become “official record[s] and subject to the law.”

Mr. Powell, apparently implying that he was cautious in his use of a personal email account, added: “Be very careful. I got around it all by not saying much and not using systems that captured the data.” According to the summary of her interview, Mrs. Clinton said that she did not know exactly what Mr. Powell was saying in that email and that his message “did not factor into her decision to use a personal email account.”

F.B.I. officials appear to have questioned Mrs. Clinton most aggressively about her judgment in using her private, unsecured system to get emails about how or where the Obama administration was planning to launch drone strikes against terrorism suspects, the documents indicated.

The F.B.I. showed her one email after another containing information about possible drone strikes that was considered classified. But Mrs. Clinton appeared almost blasé in explaining her use of her private system to gather information on drone strikes.