Washington (CNN) -- A House Intelligence subcommittee will hear from CIA security officers who are expected to tell a much more detailed story about the terror attack in Benghazi, Libya, that killed Ambassador Christopher Stevens and three other Americans last year, CNN has learned.
The men, described by sources as former Navy SEALs, former Army Special Forces and former Marines, were under contract to guard CIA agents on the ground there.
The security officers were among those who responded when Stevens' compound was attacked on the night of September 11, 2012.
They will appear before lawmakers behind closed doors during the week of November 11, sources told CNN.
Members of Congress have been trying to get access to the security officers and CIA agents, but those attempts have failed to date.
Of the estimated nearly two dozen CIA operatives on the ground that night, only one has testified, the sources said.
Frustrated lawmakers have told CNN they have been dissatisfied with the investigation so far conducted by House Intelligence Committee Chairman Mike Rogers, R-Michigan.
His staff defends the work of the committee, telling CNN previously the review has included nine full committee hearings, roundtable discussions with some Obama administration officials, an interim report and a vow that the investigation continues.
The members of Congress say they don't want information that's filtered like what they've gotten in briefings and documents, but that they want to hear the answers to three basic questions straight from those who were there:
One: What was the CIA doing in Libya? There have been allegations the CIA was operating a gun-running program with weapons going from Libya to Syrian rebels.
Two: What happened during the failed rescue attempts? CNN has been told that there was a group of would-be rescuers at the CIA annex, armed and ready to go within minutes of the attack, but they were held off until finally they defied orders and staged a rescue on their own.
Three: Did the administration know immediately that this was a planned terrorist attack? And if so, why did administration officials try to first claim it was a spontaneous response to a demonstration over a movie that offended Muslims?
"I think that there's a real mystery here surrounding what really took place and for reasons unbeknownst to me, the Congress as well as the agency are going out of their way to protect whatever there was that they were doing operationally in Libya," said Fred Burton, a former State Department diplomatic security agent who has written a book about Benghazi that's now being turned into an HBO movie.
And sources say the CIA has been trying to keep its employees quiet.
CNN reported previously that some operatives involved in the agency's missions in Libya have been subjected to frequent -- even monthly -- polygraph examinations to find out whether they've have spoken to Congress or the media, according to sources with deep inside knowledge of the agency's workings.
The CIA said in a statement this is "patently false."
"Not a single CIA officer who was on the ground in Benghazi during the attacks has been subjected to any CIA polygraph intended to discourage them from speaking to Congress or as retaliation," the agency told CNN.
"To date, some of these officers have already spoken to the oversight committees on Benghazi," it added.
CNN's Elizabeth Nunez contributed to this report.