CNN has uncovered exclusive new information about what is allegedly happening at the CIA, in the wake of the deadly Benghazi terror attack. Four Americans, including Ambassador Christopher Stevens, were killed in the wake of the deadly Benghazi terror attack. Four Americans, including Ambassador Christopher Stevens, were killed in the assault by armed militants last September last September 11 in eastern Libya. Sources now tell CNN dozens of people working for the CIA were on the ground that night, and that the agency is going to great lenghts to make sure whatever it was doing, remains a secret. CNN has learned the CIA is involved in what one source calls an unprecedented attempt to keep the spy agency's Benghazi secrets from ever leaking out. Read: Analysis: CIA role in Benghazi under-reported. 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. The goal of the questioning, according to sources, is to find out if anyone is talking to the media or Congress. 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. In other words, the rate of the kind of polygraphs alleged by sources is rare. "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 on a fishing expedition. But it's absolutely not routine at all to be polygraphed monthly, or bi-monthly," said Baer. CIA spokesman Dean Boyd asserted in a statement that the agency has been open with Congress. "The CIA has worked closely with its oversight committees to provide them with an extraordinary amount of information related to the attack on U.S. facilities in Benghazi," the statement said. "CIA employees are always free to speak to Congress if they want," the statement continued. "The CIA enabled all officers involved in Benghazi the opportunity to meet with Congress. We are not aware of any CIA employee who has experienced retaliation, including any non-routine security procedures, or who has been prevented from sharing a concern with Congress about the Benghazi incident." Among the many secrets still yet to be told about the Benghazi incident." Among the many secrets still yet to be told about the Benghazi mission, is just how many Americans were there the night of the attack.