Up to 35 CIA operatives were working in the city during the attack last September on the US consulate that resulted in the death of Ambassador Chris Stevens and three other Americans, according to CNN.

The circumstances of the attack are a subject of deep division in the US with some Congressional leaders pressing for a wide-ranging investigation into suspicions that the government has withheld details of its activities in the Libyan city.

The television network said that a CIA team was working in an annex near the consulate on a project to supply missiles from Libyan armories’ to militants in Syria.

Sources said that more Americans were hurt in the assault spearheaded by suspected radicals than had been previously reported.

CIA chiefs were actively working to ensure the real nature of its operations in the city did not get out.

So only the losses suffered by the State Department in the city had been reported to Congress.

"Since January, some CIA operatives involved in the agency's missions in Libya, have been subjected to frequent, even monthly polygraph examinations, according to a source with deep inside knowledge of the agency's workings," CNN reported.

It is being described as pure intimidation, with the threat that any unauthorized CIA employee who leaks information could face the end of his or her career.

In exclusive communications obtained by CNN, one insider writes, "You don't jeopardize yourself, you jeopardize your family as well."

Another says, "You have no idea the amount of pressure being brought to bear on anyone with knowledge of this operation."

"Agency employees typically are polygraphed every three to four years. Never more than that," said former CIA operative and CNN analyst Robert Baer.

"If somebody is being polygraphed every month, or every two months it's called an issue polygraph, and that means that the polygraph division suspects something, or they're looking for something, or they're on a fishing expedition. But it's absolutely not routine at all to be polygraphed monthly, or bi-monthly," said Baer.

Frank Wolf, a US congressman who represents the district that contains CIA headquarters in Langley, Virginia, is one of 150 members of Congress for a new investigation into the failures in Benghazi.

"I think it is a form of a cover-up, and I think it's an attempt to push it under the rug, and I think the American people are feeling the same way," he said. "We should have the people who were on the scene come in, testify under oath, do it publicly, and lay it out. And there really isn't any national security issue involved with regards to that."

A CIA spokesman said it had been open about its activities in Benghazi.

"The CIA has worked closely with its oversight committees to provide them with an extraordinary amount of information related to the attack on US facilities in Benghazi," a CIA statement said. "CIA employees are always free to speak to Congress if they want," the statement continued.