The source added the investigation doesn't hinge on getting to al Harzi alone.
The State Department has struggled with answering questions about its decision-making surrounding the attacks and said Friday that it handed over documents to Congress, which has bombarded the department with requests from committees for documents on the attack, and there could be more.
"We have now made documents available to members and staff on the House Oversight and Government Reform Committee, the Senate Homeland Security and Government Affairs Committee, the Senate Foreign Relations Committee," State Department spokeswoman Victoria Nuland told reporters.
Nuland said the department told the committees and their staff "they can see these documents as many times as they'd like to see them for as long as they'd like to see them."
One of the senators studying the documents Friday was Bob Corker, R-Tennessee, a member of the Senate Foreign Relations Committee. Corker and fellow committee member Sen. Johnny Isakson, R-Georgia, had written to Secretary of State Hillary Clinton pressing for disclosure of all communications regarding security in Benghazi leading up to the attacks.
The senators complained in their letter that cables, e-mails and other communications regarding the security situation in Benghazi "had been leaked to some Congressional offices and media outlets, resulting in conflicting reports in the press."
Nuland denied accusations that the administration was withholding information, telling reporters, "We've really done our utmost under the secretary's instructions to be fully compliant, transparent and open with the Congress."
The documents, some of which are classified, are being provided to members of the House and Senate and their staffs who, Nuland explained, view them in rooms on Capitol Hill designated for dealing with classified information.
The State Department spokeswoman also said senior officials will be on Capitol Hill next week to brief members of Congress.
Tuesday, Undersecretary of State Patrick Kennedy and Assistant Secretary of State for Diplomatic Security Eric Boswell will brief members of the Senate Foreign Relations Committee.
Wednesday, Kennedy and Boswell will brief members of the Senate Homeland Security and Government Affairs Committee.
Thursday morning, Kennedy will testify before the House Permanent Select Intelligence Committee; in the afternoon, he will testify before the Senate Select Committee on Intelligence.
Friday morning, he will brief the Chairmen and Ranking Members from the House.
All sessions will be closed to the public, Nuland said, at the request of Congress.
Clinton will not testify. Nuland said Clinton was asked by the House Foreign Affairs Committee but the State Department wrote back to the chairman to say that she will be traveling abroad next week.