Story highlights CIA has said contractors not required to sign nondisclosure forms about Benghazi

But questions are raised about the timing, intent of the forms

Contractors who responded to terror attacks in Libya tell their story to Congress

Ambassador Christopher Stevens and three other Americans were killed

CIA contract security officers who responded to the terrorist attacks on the U.S. ambassador in Benghazi, Libya, last year were told by the CIA to sign nondisclosure agreements just before a memorial ceremony at the CIA's headquarters in Langley, Virginia, this past May, a congressional source told CNN.

The source says this was disclosed by the contractors during closed-door testimony on Capitol Hill this week, and it seems to contradict information sent by CIA Director John Brennan to the House Intelligence Committee investigating the events in Benghazi.

The CIA has repeatedly denied reports by CNN that the CIA has tried to prevent its personnel from talking to members of Congress investigating the September 2012 Benghazi attacks.

When it was first reported by CNN, Brennan released a letter showing that he strongly encouraged CIA operatives to cooperate with congressional investigators. Brennan even went so far as to directly answer a question from House Intelligence Committee Chairman Mike Rogers who, in a September 3 letter to the CIA director, specifically asked this question:

"Has any officer, either staff of contractor, been required to sign any nondisclosure agreement because of their presence at Benghazi or their participation in any activity related to the Benghazi attacks?"

Brennan's answer: "No."

CIA contract operatives testifying before a classified House Intelligence subcommittee on Wednesday and Thursday directly contradicted Brennan's answer, a congressional source told CNN.

CIA spokesman Todd Ebitz told CNN that "there is absolutely no contradiction."

"CIA contractors routinely sign secrecy agreements, which are standard forms. No CIA officer has ever signed a secrecy agreement that referenced Benghazi or that prohibited them from talking to Congress," Ebitz said in a statement.

"In fact, CIA secrecy agreements specifically note an officer's right to bring issues to the attention of Congress. Furthermore, Director Brennan extended to all Benghazi survivors an invitation to speak to Congress and indicated the Agency would support their interaction. Several have spoken to CIA's oversight committees."

All five operatives were part of a security team hired by the CIA to protect its operations during missions in Libya.

The official nondisclosure forms did not specifically mention the Benghazi investigation, but the source told CNN, "There is not a person in Washington, D.C. who doesn't understand why the forms were put in front of these people."

The Weekly Standard first reported the signing of additional nondisclosure agreements on Thursday.

Mark Zaid, an attorney representing three of the CIA contractors, told the Weekly Standard, "There is no doubt that the NDAs (nondisclosure agreements) would not have been presented to them had it not been for Benghazi. That is their impression and my analysis based on 20 years of experience."

Zaid continued, "the NDAs in no way changed the legal landscape. They had already signed NDAs that bound them to certain obligations. These new ones were legally unnecessary."

The CIA contractors told to sign the nondisclosure agreements were attending a memorial service for Tyrone Woods and Glen Doherty, two CIA operatives who died in the Benghazi attacks.

This week, the House Intelligence Subcommittee on Oversight met to hear closed-door testimony from the former security agents.

Though the testimony is secret, the congressional source with inside knowledge of the hearing told CNN, "There now appears to be serious discrepancies" between what the CIA has officially reported to Congress and what the witnesses are telling members of the committee.

CNN has learned a rescue team of five CIA operatives were armed and ready to respond to the terrorist attack within minutes of the first radio communication from Ambassador Christopher Stevens' compound, but CNN sources say the agents were told to stand down.

The allegation was first raised last year, after which the CIA told Congress there was no order to stand down. The CIA has said "no one at any level in the CIA told anybody not to help those in need; claims to the contrary are simply inaccurate."

Members of the House Intelligence Subcommittee believe they need to have more information to determine why a discrepancy now exists between the would-be rescuers on the ground and the official reports from the CIA.

The agents eventually did respond to the attack, pulling off a heroic attempt to save U.S. State Department officials who were under siege.

Intelligence officials explained last year that there was a roughly 25-minute gap from the initial call for help to when half a dozen officers were able to make a move -- and that the delay was due to the time it took to load weapons and equipment and coordinate with friendly militias.