Slideshow: John Makely / NBC News Not open to the public but displayed inside CIA headquarters are artifacts from decades of intelligence gathering, including drones disguised as insects, a pigeon camera and Osama bin Laden's AK-47. Launch slideshow

Editor's note: A correction has been made to this story.

The “coolest museum you’ll never see” has a new piece de resistance – the gun found next to the body of Osama bin Laden in Pakistan when Navy SEALs killed him in a midnight raid.

The AK-47 is a recent addition to a collection that’s among the toughest tickets in the country for museumgoers. Tucked into various hallways at CIA headquarters in Langley, Va., the museum displays the gadgets, artifacts and trophies of 70 years of spycraft, from World War II through the War on Terror. The museum is closed to the public and is only visited by employees and invited guests. It is rare for cameras to be allowed in.

The Russian-made assault rifle, identified on a simple brass plaque as “Osama bin Laden’s AK-47,” shares a glass case with an al Qaeda training manual found in Afghanistan soon after 9/11.

"This is the rifle that was recovered from the third floor of the Abbottabad compound by the assault team," said curator Toni Hiley. "Because of its proximity to (bin Laden) there on the third floor in the compound, our analyst determined it to be his. It's a Russian AK with counterfeit Chinese markings."

Neither Hiley nor the agency will say how the AK-47 got to the museum, other than that the agency director at the time of the operation, Leon Panetta, "asked that it come into the museum collection," said Hiley. But one source told NBC News that it came from the "dark side" of the agency, the operations staff that worked with the SEALs on the May 2011 raid.

John Makely /NBCNews.com CIA museum curator Toni Hiley holds an "insectothopter" created by the CIA's Office of Research and Development during the 1970s and intended to gather intelligence unobtrusively. Designed to look like a dragonfly, the insectothopter's tiny gas-powered engine moved its wings up and down. While flight tests were impressive, it proved difficult to control when any wind was present.

The agency also will not comment on the specifics of how the weapon was recovered or whether it was loaded when retrieved.

"I wasn't there," said Hiley. "So I can't confirm or deny exactly where the weapon was. I just know that I have it in my museum and I'm happy to have it."

In the movie "Zero Dark Thirty," which was written in consultation with military and intelligence sources, a member of the assault team is shown grabbing the weapon from a shelf above bin Laden's bed in his third-floor bedroom moments after the al Qaeda leader’s death.

Hiley said the weapon is in good working condition, but that the origin of the Chinese markings is a mystery. She said it’s not the weapon seen at Osama’s side in many propaganda videos.

The CIA’s private museum, which was started in the early 1990s, fills three corridors in two buildings at the CIA campus just outside Washington. Agency officials call it “the coolest museum you’ll never see.”

Filled with secrets, the CIA museum -- which is closed to the public – houses all kinds of artifacts: from a 70s-era drone, to the gun found next to Osama bin Laden when Navy SEALs killed him in a midnight raid. NBC's Richard Engel reports.

The museum traces the agency’s history, including its origin as the Office of Special Services (OSS), which aided resistance fighters and ran spy networks during World War II, and its years of clandestine operations during the Cold War. Artifacts include the shrapnel that struck a spy plane over North Vietnam, a silver dollar that holds microfilm and an underwater spy drone made to look like a catfish. The museum previously displayed many of the phony Hollywood accoutrements – a movie script, stationery, and company briefcase -- that a team of agents posing as filmmakers used in an audacious operation that rescued six Americans from Iran in 1979. The mission became the basis of the recent Academy Award-winning movie “Argo.”

Bin Laden’s gun is part of a new trove of artifacts, the spoils of the War on Terror. "I think for our people it's an acknowledgment that the hard work over that 10 years and partnership with other members of the intelligence community and partnership with the military was a success,” said Hiley. “This puts the punctuation point on 10 years of this agency and our intelligence community partners looking for bin Laden."

In addition to bin Laden’s gun, there's a brick from his compound in Abbottabad; a scale-model of the compound; a section of a wall that was part of the life-size mock-up of the compound used by the SEALs to train for the raid; and several al Qaeda training manuals found in Afghanistan, including a partially burned guide to firing surface-to-air missiles.

Tony Hiley, Director of the CIA Museum, shows NBC's Richard Engel what she considers one of the most unique pieces in the museum, the A-12 Oxcart plane, which was designed to be the successor to the U-2 spy plane.

There’s also a chest filled with football-sized chunks of blue and white lapis lazuli stones, one of a dozen chests of the rare stones seized by the CIA in Afghanistan. Hiley referred to the trunks as "al Qaeda's ATM." Al Qaeda used the raw lapis lazuli, which goes for $200 to $1,200 a kilogram, to circumvent banks and pay its fighters.

The scale-model of the compound is identical to one the raid’s planners viewed in Director Panetta's office and the White House Situation Room. (The original is at the Pentagon.)

"It's as accurate as those hundreds of pieces of intelligence would permit," said Hiley, when asked if the spare tires in the yard matched intelligence reports. "Not only are they looking at that intelligence, they're going to the analysts and asking them, 'Was the wire this far apart?' So they're trying to get absolutely every single bit of truth into this model because they know the model will be used."

Nearby is a piece of the wall, topped with barbed wire, from a full-scale mock-up that the CIA constructed for SEAL training. The life-size mock-up was destroyed not long after the May 2011 raid.

NBC News NBC News Chief Foreign Correspondent Richard Engel interviews CIA museum curator Toni Hiley at CIA headquarters in Langley, Va.

As Hiley explained, the SEAL team planned its raid using the scale model, and then practiced the raid on the life-size mock-up. "I think we received the best feedback we ever could have hoped to when they all returned safely,” said Hiley. “They said, ‘We felt like we'd been there before.’”

“That's why these pieces are important. That's why they're in this museum."

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Behind the scenes at the CIA museum

Some images from the collection are viewable online via the agency’s website, so while the museum is off-limits to the public most of its contents are not unknown. There is, of course, another secret museum behind the not-so-secret” museum, a “classified collection” kept in a secret warehouse, from which curators will sometimes pull new items to display.

Asked if this classified collection was more like a police evidence locker or the basement of the Smithsonian, Hiley said it was like neither.

"Oh," she said, "it's much cooler than that."

Part 2: Among CIA museum's prizes, an American love letter on Hitler's stationery.

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