By CNN's Elise Labott and Dana Bash

At least four State Department and CIA employees are being intimidated and blocked from cooperating with a congressional investigation into the deadly terror attack on the U.S. diplomatic mission in Benghazi, Libya, last year, according to an attorney for one of the officials.

The officials consider themselves whistleblowers and feel threatened with career damage if they decide to give testimony to Congress, according to Victoria Toensing, an attorney for one of the State Department officials.



The story was first reported by Fox News.

"If you are going to take away somebody's job or living then it's a threat," Toensing said.

She would not go into specifics about potential threats her client might have faced. But she said that "it's done in a more subtle way" where suggestions are made that plum assignments will not be available for those who might testify.

Under federal law, employees identified as "whistleblowers" are protected from repercussions by their employer for giving damaging testimony about a government agency to Congress or an inspector general, an independent investigator within an agency.

Toensing, a former Justice Department official and Republican counsel to the Senate Intelligence Committee, would not name her client or confirm whether they were on the ground the day of the September 11, 2012, attack on the diplomatic facility and CIA annex.

Ambassador Christopher Stevens and three other Americans were killed.

Toensing said her client had relevant information to share with Congress about the attack itself as well as the months leading up to it when requests for more security by Stevens and his staff were largely refused. She also said there was information about the administration's controversial characterization of the attack in the days following.

But Toensing said her client was unable to discuss classified information and key evidence in the case because she (Toensing) didn't have the appropriate security clearance.

"There is a clear obstruction to my client when my client cannot give me all the information because the State Department will not give a process for my being cleared," Toensing said. "What the State Department has to do is clear the lawyer for the information to come out. So even if my client is a witness they will only get half a story."

President Barack Obama on Tuesday said he was unaware of charges that State Department and CIA employees have been intimidated.

"I'm not familiar with this notion that anybody's been blocked from testifying," Obama said in a response to a question at a White House news conference.

In a letter to Secretary of State John Kerry, Rep. Darrell Issa of California complained the State Department needed to establish a process by which attorneys could be provided necessary security clearances to review classified information.

"It is unavoidable that department employees identifying themselves as witnesses in the committee's investigation will apply for a security clearance to allow their personal attorneys to handle sensitive or classified material," the Republican chairman of the House Oversight and Government Reform Committee said.

"The department's unwillingness to make the process for clearing an attorney more transparent appears to be an effort to interfere with the rights of employees to furnish information to Congress," he added.

The State Department said it was unaware of any employees who have requested security clearance for private attorneys in connection with the Benghazi matter, adding that department does in fact have a process in place for gaining such clearances.

"The State Department would never tolerate or sanction retaliation against whistleblowers on any issue, including this one," deputy agency spokesman Patrick Ventrell said.

Toensing said no employees have come forward asking for clearance for their lawyers because they are afraid of reprisals.

"I have to protect my client and I am not going to let my client go to people in the State Department and expose himself or herself without my being able to be with that person and if I am not cleared, I can't be with that person," she said. "So it's a chicken and egg thing."

When asked about the issue, Kerry told reporters, "there's been an enormous amount of misinformation out there."

Kerry vowed to cooperate with Congress, but added, "We have to demythologize this issue, and certainly depoliticize it."

The Obama administration continues to face criticism from Republicans over security lapses in Benghazi leading up to the attack. Its early account that it was the result of a spontaneous protest turned out to be wrong.

On Tuesday, Republican Sens. Lindsey Graham of South Carolina and John McCain of Arizona reiterated their call for the Senate to establish a special committee to investigate the administration's handling of the attack.

"Revelations about witnesses being afraid to testify and military assets that could have been deployed in a timely fashion justify appointing a joint select committee," they said in a statement.

Earlier this month, Kerry suggested lawmakers were harping on the administration's response and that it was time to move on.

"Let's get this done with, folks," Kerry told the House Foreign Affairs Committee in his first appearance before Congress since taking office. "I do not want to spend the next year coming up here talking about Benghazi."

Kerry maintained the administration has been more than forthcoming with Congress over Benghazi. Administration officials involved in the matter have testified eight times before Congress, briefed congressional leaders roughly 20 times and submitted about 25,000 pages of related documents.

"And that should be enough," Ventrell told reporters this week. "Congress has its own prerogatives, but we've had a very thorough, independent investigation, which we completed and [which was] transparent and shared. And there are many folks who are, in a political manner, trying to sort of use this for their own political means, or ends."