WASHINGTON — In 2012, congressional investigators asked the State Department for a wide range of documents related to the attack on the United States diplomatic compound in Benghazi, Libya. The department eventually responded, furnishing House committees with thousands of documents.

But it turns out that that was not everything.

The State Department had not searched the email account of former Secretary of State Hillary Rodham Clinton because she had maintained a private account, which shielded it from such searches, department officials acknowledged on Tuesday.

It was only last month that the House committee appointed to investigate Benghazi was provided with about 300 of Mrs. Clinton’s emails related to the attacks. That was shortly after Mrs. Clinton turned over, at the State Department’s request, some 50,000 pages of government-related emails that she had kept on her private account.

It was one of several instances in which records requests sent to the State Department, which had no access to Mrs. Clinton’s emails, came up empty.