WASHINGTON — The Chinese spy balloon that floated across the continental United States before it was shot down by the U.S. military generated deep concern on Capitol Hill in part because it came on the heels of a classified report that outlined incidents of U.S. adversaries potentially using advanced aerial technology to spy on the country.

The classified report to Congress last month discussed at least two incidents of a rival power conducting aerial surveillance with what appeared to be unknown cutting-edge technology, according to U.S. officials. While the report did not attribute the incidents to any country, two U.S. officials familiar with the research said the surveillance probably was conducted by China.

The report on what the intelligence agencies call unidentified aerial phenomena focused on several incidents believed to be surveillance. Some of those incidents have involved balloons, while others have involved quadcopter drones.

The Chinese government said Friday that the Chinese balloon discovered over the United States was mainly for weather research. However, U.S. officials said they have assessed it to be a collection device, although not one that could gather the kind of sensitive information that advanced Chinese reconnaissance satellites already collect.

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Many countries use aerial spying technology to gather data on rival nations as well as allies and partners, and to look at remote parts of the globe. But the practice can lead to diplomatic crises and greater military tensions when it goes awry.

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On Friday, Secretary of State Antony Blinken canceled a weekend trip to Beijing, which would have been the first visit by the top U.S. diplomat there since October 2018, after American news organizations began reporting on the Chinese spy balloon on Thursday, when it was drifting over Montana. On Saturday, the U.S. military shot down the balloon after it entered skies over the Atlantic Ocean.

In 2001, a U.S. Navy signals intelligence aircraft collided with a Chinese interceptor jet near the Chinese island of Hainan; the incident left a Chinese pilot presumed dead and led to a diplomatic crisis involving the leaders of the two nations.

China spends about $209 billion, or 1.3% of gross domestic product, on its military overall, according to a Pentagon report. But policymakers in Washington have been especially worried about its investments in technologies that could have military or intelligence applications.

U.S. defense officials believe China is conducting surveillance of military training grounds and exercises as part of an effort to better understand how America trains its pilots and undertakes complex military operations. The sites where unusual surveillance has occurred include a military base in the United States and a base overseas, officials said.

The classified report mentioned Naval Air Station Fallon in Nevada and Marine Corps Air Station Iwakuni in Japan as sites where foreign surveillance was believed to have occurred, but did not explicitly say China had been behind the actions, a U.S. official said.

Since 2021, the Pentagon has examined 366 incidents that were initially unexplained and said 163 were balloons. A handful of those incidents involved advanced surveillance balloons, according to a U.S. official, but none of them were conducting persistent reconnaissance of the U.S. military bases. (However, spy balloons that the U.S. government immediately identifies are not included in unidentified aerial phenomenon tracking, according to two U.S. officials.)

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Because spy balloons are relatively basic collection devices and other balloons have not lingered long over U.S. territory, they previously have not generated much concern with the Pentagon or intelligence agencies, according to two officials.

The surveillance incidents involving advanced technology and described in the classified report were potentially more troubling, involving behaviors and characteristics that could not be explained.

Officials said that further investigation was needed but that the incidents could potentially indicate the use of technology that was not fully understood or publicly identified. Of the 171 reports that have not been attributed to balloons, drones or airborne trash, some “appear to have demonstrated unusual flight characteristics or performance capabilities, and require further analysis.”

Even outside the incidents mentioned in the classified report, some current and former military officials warned against underestimating the advanced surveillance technology that could be embedded in the Chinese spy balloon. Pentagon officials say the belly section of the balloon that houses surveillance equipment is about 90 feet long, or equivalent to three school buses.

“There’s more of a potential risk from these kinds of balloons than many people think,” said Gen. Victor E. Renuart Jr., a former head of the North American Aerospace Defense Command, or NORAD, which carries out the air defense mission within the U.S. military’s Northern Command.

Officials cautioned that imperfect radar and sensor readings could cause confusion, making an ordinary object appear to be something threatening.

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The Pentagon has released images of some of the suspected surveillance incidents that were initially unexplained. The images showed green triangles in the air taken near two different Navy exercises. At a congressional hearing last year, Pentagon officials said the triangles were simply small drones. The use of night-vision gear had made them look otherworldly.

While the drones had not been officially attributed to any country, in one incident a Chinese ship was in the vicinity.

It is not clear how strong the evidence is that China is using an advanced technology that the United States does not possess. Some U.S. officials remain skeptical that China would risk exposing some of its most advanced technology in any surveillance activity that could be detected by the United States.

The surveillance balloon stirred outrage on Capitol Hill. Some officials said the information about adversarial spying contained in the classified report on unidentified aerial phenomena had already driven up concern earlier.

Both Republicans and Democrats hawkish on China called the surveillance balloon a violation of U.S. sovereignty that highlighted the threat from Beijing.

Rep. Mike Gallagher, R-Wis., a member of the House Intelligence Committee and chair of a new House committee on China, said the administration needs to tell lawmakers more about what it knows about surveillance of military facilities.

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“This is all the more reason for the House Intelligence Committee to receive a full briefing on this matter,” Gallagher said Friday. “There is a documented history of unidentified — and now identified — objects near sensitive military facilities, and we need to move with a sense of urgency to get to the bottom of this.”

When China tested a hypersonic missile in 2021, Gen. Mark Milley, chair of the Joint Chiefs of Staff, warned that it was something close to a Sputnik moment, referencing the Soviet Union’s successful Cold War satellite launch. Hypersonics has been a key technological focus of Beijing and is one area where China has demonstrated capabilities equal or exceeding those of the U.S. military.

But the incidents potentially involving advanced technology described in the classified report are not believed to involve any sort of hypersonic propulsion, U.S. officials said, nor does the Chinese spy balloon that generated a diplomatic crisis.

“We don’t need any more evidence than this that we are in a Cold War-like long-term competition that will be expressed in military and intelligence terms for the coming decade,” Evan Medeiros, a Georgetown University professor and senior Asia director on the White House National Security Council in the Obama administration, said of the balloon episode involving China. “And what the relationship lacks is the mechanisms to manage this.”