President Obama said he is unaware of longstanding efforts by Republican lawmakers to question survivors of the Benghazi attacks but pledged to investigate the issue.

“I’m not familiar with this notion that anybody has been blocked from testifying,” the president said during a White House news conference on Tuesday. “So what I’ll do is I will find out what exactly you’re referring to.”

Obama’s pledge to find out more came as officials at the State Department pushed back against allegations -- first aired Monday on Fox News -- that career employees at the agency have been threatened if they furnish new information about the Benghazi attacks to members of Congress.

“The State Department is deeply committed to meeting its obligation to protect employees, and the State Department would never tolerate -- tolerate or sanction -- retaliation against whistle-blowers on any issue, including this one,” spokesman Patrick Ventrell told reporters at a briefing on Tuesday. “That’s an obligation we take very seriously -- full stop.”

Four Americans, including U.S. Ambassador to Libya Chris Stevens, were killed in terrorist attacks on U.S. installations in the port city of Benghazi, Libya, on the night of Sept. 11, 2012. While the FBI investigation into the attacks continues, no known instances of any perpetrators being brought to justice have yet been reported.

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Rep. Darrell Issa, R-Calif., chairman of the House Oversight and Government Reform Committee, will hold the first in a new round of hearings on the subject on May 8.

In two letters to the State Department, dated April 16 and April 26, Issa has sought explicit guidance on how attorneys representing witnesses with knowledge of the Benghazi attacks, including their prelude and aftermath, can receive the security clearances necessary to review classified materials.

“Attorneys representing Department personnel in this matter will require clearance to possess and discuss Top Secret and Sensitive Compartmented Information,” Issa wrote on April 16 to Mary McLeod, the principal deputy legal adviser to the State Department.

But Ventrell insisted Tuesday that no such whistle-blowers have come forward, and no requests for security clearances have been made by private attorneys.

Victoria Toensing, a former Justice Department official and one-time Republican counsel to the Senate intelligence committee, disclosed on Monday that she is representing a career State Department official who identifies himself as a whistle-blower. Toensing said this individual has been threatened by superiors with career-ending reprisals if he cooperates with the oversight committee.

“[The State Department has] had two letters from Chairman Issa, one on April 16, the other one April 26, that specifically say, ‘We want you to provide a process for clearing a lawyer to receive classified information,’” Toensing said during an interview Tuesday on “America’s Newsroom” with Fox News anchor Martha MacCallum. “How can they possibly get up there and just lie to the press corps?”

Ventrell said that the State Department periodically sends out notices to the entire staff advising them of the protections afforded whistle-blowers under federal law, and that such a notice, in accordance with regular practice every spring, was disseminated just last week.

Interviewed on the Los Angeles campus of the University of Southern California on Tuesday, Sen. John McCain, R-Ariz., argued the allegations of threats and intimidation expose the need for a more comprehensive probe of the Benghazi affair.

“People do not trust the president and his people,” McCain told Fox News. “That's why we need a select committee.”

Fox News’ Martha MacCallum and Lee Ross contributed to this report.