U.S. intelligence agencies were aware of up to four additional Chinese spy balloons, and questions lingered about the true capabilities of the one that flew over the continental United States in January and February, according to previously unreported top secret intelligence documents.

The Chinese spy balloon that flew over the United States this year, called Killeen-23 by U.S. intelligence agencies, carried a raft of sensors and antennas the U.S. government still had not identified more than a week after shooting it down, according to the document allegedly leaked to a Discord chatroom by Jack Teixeira, a member of the Massachusetts Air National Guard.

The balloon was one of at least three, including another that flew over a U.S. carrier strike group in a previously unreported incident and a third that crashed in the South China Sea, a second top secret document stated, though it did not provide specific information for launch dates.

One of the three documents, produced by the National Geospatial-Intelligence Agency (NGA) and dated Feb. 15 — 10 days after the Air Force shot down the balloon that flew over the United States — contains the most detailed government assessment to date of Killeen-23 and two balloons from previous years, labeled Bulger-21 and Accardo-21. It was not clear from the documents if Bulger-21 and Accardo-21 were the same balloons that flew over the carrier strike group and crashed.

Bulger-21 carried sophisticated surveillance equipment and circumnavigated the globe from December 2021 until May 2022, the NGA document states. Accardo-21 carried similar equipment as well as a “foil-lined gimbaled” sensor, it says.

A U.S. official speaking on the condition of anonymity to discuss sensitive intelligence said the government naming convention for such balloons is alphabetical, from A to Z. It appears that the balloons are named after notorious criminals, including Tony Accardo, James “Whitey” Bulger, and Donald Killeen.

Annotating what appear to be detailed photos of the balloon that flew over the United States, presumably taken from a U-2 spy plane, intelligence analysts assessed that it could generate enough power to operate “any” surveillance and reconnaissance technology, including a type of radar that can see at night and through clouds and thin materials.

The overflight of the carrier strike group will raise questions in Congress, where Republicans seized on the spy balloon issue to accuse President Biden of failing to deter Chinese espionage.

The Pentagon and Office of the Director of National Intelligence declined to comment.

The incursion of the spy balloon into U.S. airspace in late January likely caught elements of China’s government by surprise, according to a third document that relies on intercepted communications. Knowledge of the incursion was likely “heavily stovepiped” within the Chinese military, which lacks “strong senior” oversight of the surveillance balloon program, the document assessed, adding that some in the Chinese government viewed their Foreign Ministry’s response as poor for allowing the crisis to be “sensationalized.”

The Washington Post obtained the documents from a trove of images of classified files posted on Discord, a group chat service popular with gamers. The document is part of a new tranche that has not been previously reported.

The leaked NGA document contains an image taken by Bulger-21 that appears to directly connect such a balloon to Eagles Men Aviation Science and Technology Group, one of six Chinese companies sanctioned by the United States in February for supporting the spy balloon program.

China’s military has operated a vast surveillance balloon project for several years, partly out of Hainan province off China’s south coast, U.S. officials have previously told The Post.

But the NGA document is notable as much for what it doesn’t say, reflecting the government’s possible lack of insight, at least in mid-February, into the balloons’ capabilities. The government recovered debris from the balloon’s crash site in the Atlantic Ocean, but has declined to say how much of the payload it recovered.

The leaked NGA document notes that Killeen-23 contained a parabolic dish measuring 1.2 meters in diameter, several unidentified sensors, and a possible mast antenna. The document says that the government has “no imagery collections of the bottom of the Killeen-23 payload to analyze for an optical sensor.”

The lack of detailed conclusions about the balloon’s surveillance capabilities raises questions about the decision to let it fly over the United States before shooting it down, an action the Defense Department justified at the time as an opportunity to collect additional intelligence.

Engineers at the National Space Intelligence Center, which is affiliated with the Space Force, assessed that the solar panels on Killeen-23 could generate upward of 10,000 watts of solar power, more than enough to operate any surveillance capability, including synthetic aperture radar, according to the document.

Synthetic aperture radar, or SAR, sends pulses of microwaves at the Earth to create images. Unlike traditional optical sensors, this allows SAR to return images at night and to penetrate clouds, smoke, topsoil, ice and snow. SAR has also been shown to penetrate thin materials, including tarps, revealing objects beneath.

The amount of power the Chinese balloon could generate was “humongous,” said Paul Byrne, an associate professor at Washington University in St. Louis and a specialist in remote sensing. It was about 100 times that generated by balloons such as Google’s Loon, which provides internet service, and nearly twice that generated by some orbital SAR satellites.

But the SAR data the balloon could have acquired would not have been much different from that of a satellite, Byrne said, which suggests it probably carried another imaging system, such as a camera, that would gain an advantage. The document notes that Bulger-21 carried a “full motion video payload with a capability to zoom.” Balloons offer a “potent way to conduct” surveillance, Byrne said, because they can gain higher-resolution data and their launch is “quieter” than putting a satellite into space.

Chinese military researchers have for years written about the advantages of deploying SAR capabilities on “near space” vehicles. An article published in the official newspaper of the Chinese military, the PLA Daily, in September 2020 described how airships and balloons could be loaded with radar and photoelectric surveillance equipment to detect targets for strikes.

The leaked NGA document also contains a close-up image of an “unidentified structure” measuring about .75 meters in diameter protruding from the top of Killeen-23, the purpose of which is unclear.

A photograph in the document described as a “commercial vendor image” from Bulger-21 contains the logo of the Eagles Men Aviation Science and Technology Group and text that says Static Stratospheric Constellation System in Chinese, a reference to networks of balloons that could operate as an alternative to satellite-based communications and observation.

It was unclear where the image was obtained or what system it described, but it appeared to be the first direct link between the sanctioned company and a spy balloon.

The Post was unable to confirm images of Bulger-21 in cached versions of the Eagles Men company website, which was taken down shortly after the company was sanctioned.

Neither document gives a name or date for the balloon that flew over the U.S. carrier strike group. The Nimitz Carrier Strike Group was positioned in the western Pacific in January and February, according to public government information. The government notified Congress in February of several previous incursions into U.S. airspace by Chinese spy balloons, including near Texas, Florida, Hawaii and Guam.

The Defense Department declined to comment about the overflight of the carrier strike group.

Pei-Lin Wu in Taipei, Taiwan, contributed to this report.

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