Project SIGN: The 1947–48 File

The Project SIGN file is a 245-page declassified correspondence file from the US Air Force's Directorate of Intelligence, released on 7 August 2026, covering the creation and first year of the military's first official UFO investigation. It contains the September 1947 Twining memo calling the phenomenon "something real", the December 1947 order creating Project SIGN, an intelligence analysis concluding that if the discs existed they were "foreign in origin", and the November 1948 letters in which the Air Force privately admitted it had weighed the possibility of "vehicles from another planet" while advising that the press be told about balloons and stars.

Some press coverage has called this document "an Air Materiel Command report on UFOs". It is not a report. It is the file a headquarters keeps: letters going down, indorsements coming back up, routing slips, memoranda for record, and the occasional letter from a member of the public. That makes it slower reading than a report, and considerably more revealing. Reports are written to be shown to someone. Files are what an organisation says to itself.

What Was Project SIGN?

Project SIGN was the US Air Force's first formal UFO investigation, ordered on 30 December 1947 and run from Air Materiel Command at Wright-Patterson Air Force Base in Dayton, Ohio. The order, signed by General Craigie, instructed the command to "set up a project whose purpose is to collect, collate, evaluate and distribute to interested government agencies and contractors all information concerning sightings and phenomena in the atmosphere which can be construed to be of concern to the national security". SIGN ran through 1948 before being renamed Project GRUDGE, which in turn became Project Blue Book, the programme most people have heard of.

What this file adds is the paper trail behind that bare history. The summer 1947 flying disc wave produced thousands of newspaper stories and a real problem for the Air Force: its own pilots, radar operators and intelligence officers were among the witnesses. The file shows the machinery grinding into motion, from the first considered opinion in September 1947 to a fully argued position fourteen months later. Nothing in it was written for the public. All of it can now be read at our UAP document archive or downloaded from the Pentagon directly.

What Does the Twining Memo Say?

The Twining memo of 23 September 1947 is the earliest document in the file and the foundation of everything that followed. Asked for Air Materiel Command's considered opinion on the "flying discs", Lieutenant General Nathan Twining replied with a short, blunt assessment agreed between the command's intelligence staff and its aircraft, power plant and propeller laboratories.

The memo describes a consistent object: a metallic or light-reflecting surface, circular or elliptical in shape, flat on the bottom and domed on top, usually silent, flying level at speeds above 300 knots and reported in formations of three to nine. Then comes the sentence that still reads strangely in an official letter from 1947.

Twining was careful about what he did not have: the memo notes the lack of physical evidence "in the shape of crash recovered exhibits which would undeniably prove the existence of these objects". His recommendation was a classified, priority, code-named study with complete data sets circulated to the Army, Navy, Atomic Energy Commission, RAND and the Scientific Advisory Group. Three months later, that study existed. It was called SIGN.

The December 1947 Analysis: Six Checks, One Conclusion

The most substantial single document in the file is an 18 December 1947 staff paper, "Analysis of 'Flying Disc' Reports", prepared inside the Directorate of Intelligence. Before drawing any conclusion, its authors ran six separate checks on the obvious mundane explanations. The file records each one and its result.

Having run those checks, the analysts wrote down where the logic left them. The discs "probably represent something real and tangible", and considering their reported conduct "it must be conceded a possibility that they may have been under control, either manually or remotely, or by pre-set automatic controls". Then the conclusion that gives the paper its edge.

"Foreign" in December 1947 meant, in practice, Soviet. But the same paper had just spent a page explaining why foreign reconnaissance made little sense. The analysts left that contradiction standing. Twelve days later, Project SIGN was ordered into existence to resolve it.

Which 1947 UFO Sightings Are in the Project SIGN File?

The December 1947 analysis includes a case annex, and it is a snapshot of the first summer of the UFO era as the Air Force itself recorded it. Kenneth Arnold's famous sighting near Mount Rainier is there, and the analysts matched his sketch to photographs taken by William Rhodes over Phoenix, Arizona on 7 July 1947, which showed an object with "a round front and square tail in plan form". The file also holds sea and radar cases that rarely get told.

Worth saying plainly: this is a headquarters file, not the casebook. Roswell is not in it, and neither are Mantell, Chiles-Whitted or the other famous early Air Force cases. Anyone claiming those files are "in the Project SIGN release" has not read it.

"Vehicles From Another Planet": The Cabell and McCoy Letters

The file ends with its most quoted documents. On 3 November 1948, Major General C. P. Cabell, Director of Intelligence at HQ USAF, wrote to Air Materiel Command demanding conclusions. His letter concedes the central fact in one sentence.

Cabell's other worry was the press. Reporters were "about to take it into its own hands and demand to be told what we do or do not know", he warned, and "silence on our part will not long be acceptable". He wanted to know what, if anything, should be offered to them.

The reply came back in five days, signed for the commanding general by the chief of the intelligence department, Colonel McCoy. It reports roughly 180 incidents studied, sorted into four classes: flat discs, wingless torpedo or cigar shapes, spheres or balloons, and "balls of light". It records that Nobel laureate Irving Langmuir had been consulted and would be again. And it makes two statements that sit oddly together. First: "All information that has been made available to this Headquarters indicates that the discs, the cigar shaped objects, and the 'balls of light' are not of domestic origin." Second, the sentence this file will always be known for.

McCoy immediately added that "tangible evidence to support conclusions about such a possibility are completely lacking". But the letter shows the hypothesis was tested, not waved away. His staff plotted sighting frequency against the close approaches to Earth of Mercury, Venus and Mars and found "a periodic variation in the frequency of incidents, which appears to have some relation to the planet approach curves", though "it may be purely a coincidence". He even cites The Books of Charles Fort to note that similar phenomena had been reported for a century or more. Then, in the final paragraphs, the public-facing advice: if the press insisted on a statement, tell them many of the objects had been identified as weather balloons or "astral bodies".

What Nobody Has Reported From the Project SIGN File

The famous quotes have been in circulation since the Blue Book era. What makes this release valuable is the connective tissue around them, and several episodes in it have had no press coverage at all.

The Air Force asked for interceptors on alert, and was told no. In February 1948 the Directorate of Intelligence requested that fighters fitted with gun cameras be placed on continuous alert at every air base in the United States and Alaska, ready to intercept and photograph the discs. The refusal, dated 3 March 1948, is startling for what it admits: "Proper interception is not possible", because the Air Force "is incapable of providing such [radar] coverage" of its own territory. In 1948 the United States could not reliably see what was flying over it, and put that in writing.

RAND was asked to define a spaceship. To screen sightings against the extraterrestrial hypothesis properly, RAND was formally tasked with working out the parameters an "experimental spaceship" would exhibit. The tasking carries its own hedge, suggesting the answer was "more likely" to involve a foreign nation "rather than a product from beyond the Earth". Even so: a federal contract to specify what a spaceship would look like, in 1948.

The CIA and the red spider from Mars. In early 1948 the CIA was quietly sent to Amazing Stories magazine, which was preparing a flying disc article, to obtain its UFO correspondence. The Agency duly reported back. The magazine's letters came from "school children, illiterates" and "crackpots". The planned article's author claimed the discs came from an underground city, which he had visited. And the publisher had cancelled the article "because it is not amazing enough", preferring a story about a giant red spider from Mars that abducted humans for a Martian museum. This is documented CIA involvement in the UFO question five years before the Robertson Panel, and it ended in farce.

The rest of the file is quieter but telling. A Navy officer, Lieutenant Commander Marcus L. Lowe Jr, reported a yellow sphere on 30 April 1948 moving against the wind toward restricted airspace over Washington DC. Intelligence officers spent months trying to establish where the Horten brothers and their flying-wing designs had ended up. Reporting machinery was built out through JANAP 146, the Air Transport Association was enlisted so airline pilots could report sightings directly to Wright-Patterson, and essential-elements-of-information lists went to the Navy, Army and Coast Guard. And ordinary citizens wrote in, including a Mrs Ruth Shook, whose letter contains the most sensible sentence in the file: "If they are American military machines I do not need to know about them."

What the Project SIGN File Adds Up To

Read end to end, the file shows a fourteen-month gap opening between what the Air Force said internally and what it prepared to say publicly. Internally: the phenomenon is real, it is probably not ours, the FBI found no hoax, the scientists could not explain all of it, and the extraterrestrial hypothesis was examined seriously enough to involve RAND and a planet-approach plot. Publicly: weather balloons and astral bodies. Both positions were written by the same officers in the same file, sometimes in the same week.

The parallel with the present is hard to miss. In 2026, official releases describe objects tracked by military sensors that remain unexplained, while public statements stay carefully unexciting. That is roughly where the Air Force stood in November 1948. The difference is that we can now read the 1948 file, and it took 78 years to arrive. The green fireballs that began appearing over New Mexico's nuclear laboratories a month after McCoy's letter would run the same pattern again — see our Green Fireball Incidents page for what happened next.

Read the Project SIGN File Yourself

The full 245-page file is public. It is listed as DOW-UAP-D100, "Air Materiel Command Report on UFOs, 1947–1948", among the Department of War's UAP releases at war.gov/ufo. Ignore the title: as this page has hopefully shown, it is a correspondence file, and it rewards reading in order. Our UAP document archive tracks every release, and our UAP research hub covers the cases the documents describe. For what was in the rest of this release, start with our fifth release round-up.

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