Sen. Lindsey Graham says he is not backing down down on a threat to hold up all White House nominations for federal government positions until survivors of last year's deadly attacks on a U.S. diplomatic outpost in Benghazi, Libya, appear before Congress.

"I'm not trying to solve a crime. I'm trying to find out [what happened] from the mouths of the people who are on the ground," the South Carolina Republican told "Fox News Sunday."

"I don't think it's over the top to find out what happened to four dead Americans."

Prominent nominations announced by President Obama and awaiting Senate confirmation include Janet Yellen for chair of the Federal Reserve, Jeh Johnson for secretary of the Homeland Security Department and two nominees for the influential D.C. Circuit Court of Appeals.

The Obama administration has said it can't make witnesses available to Congress yet because its review of the Sept. 11, 2012 attacks, which killed U.S. Ambassador J. Christopher Stevens and three other Americans, is an ongoing criminal investigation.

White House spokesman Jay Carney has said the administration has made “extraordinary efforts” to cooperate with congressional committees investigating the attacks, including participating in 13 congressional hearings, 40 staff briefings and providing more than 25,000 pages of documents.

But Graham said he wants to hear directly from those who were in Benghazi before, during and after the attacks.

"A year later, only one survivor in Benghazi has been interviewed by the Congress, and that person was subpoenaed," he said. "I want to know from their mouth — not anybody else, no spokesman, no British contractor, Americans on the ground in Benghazi — did you see a protest? Did you ever report a protest?"

The administration has said the attacks were spontaneous and linked to protests earlier that day in Cairo against an American-made video denigrating Islam's prophet Muhammad. But many Republicans have speculated the attacks may have been planned in advance by al Qaeda terrorists.

Graham said will ask his Republican colleagues to "stand up to the Obama administration" so as not to "let them get away with this," he said

"I shouldn't have to do this. I shouldn't have to make these kind of threats," he said. "Can you imagine if this was George W. Bush and he told the Congress after 9/11, you can't talk to anybody because there's a potential criminal investigation?""