Defense Secretary Leon Panetta is expected to testify at an upcoming Senate hearing on the Libya terror attack, a source tells Fox News -- after Republican Sen. Lindsey Graham threatened to hold up Chuck Hagel's confirmation to replace Panetta over the issue. 

At the start of Hagel's confirmation hearing Thursday, Senate Armed Services Committee Chairman Carl Levin, D-Mich., announced that the panel would hold a hearing on Benghazi next month. He did not specify whether Panetta would testify, but a committee aide told Fox News he is expected to. 

Another aide said Graham is "happy as a clam" that his condition was met. 

Graham has been among the most vocal and persistent Republican lawmakers in calling for answers about the Sept. 11, 2012, attacks on a U.S. outpost that killed four Americans, including a U.S. ambassador. 

"I'm going to block Hagel from going forward until (Panetta) does," the South Carolina senator said Monday night on Fox News. "Why would we not want to understand what happened during the attack?" 

Graham and other Republican lawmakers continue to seek answers about the incident -- after seeing a State Department report on the attack and questioning Secretary of State Hillary Clinton for roughly 10 hours during open Capitol Hill testimony. 

Graham has also been highly critical of Hagel. 

Hagel, a former Nebraska Republican senator, was nominated by President Obama and faced tough questions during the Senate confirmation hearing about his positions on Iran and Israel. 

Graham said his decision on Hagel will depend on what he says during the hearings in the Democrat-controlled Senate, "but the one thing I'm not going to do is vote on a new secretary of Defense until the old secretary of Defense, Leon Panetta, whom I like very much, testifies." 

Among Graham's lingering questions are what happened during the seven-hour attack on the Benghazi outpost and what was Obama's response. 

Fox News' Mike Emanuel contributed to this report.