Classified U.S. Report Highlighted Foreign Power Surveillance With Advanced Technology

WASHINGTON — The Chinese spy balloon floating over the continental United States generated deep concern on Capitol Hill in part because it came on the heels of a classified report to Congress that outlined incidents of American adversaries potentially using advanced 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 American officials familiar with the research said the surveillance probably was conducted by China.

The report on what the intelligence agencies call unidentified aerial phenomenon 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 on Friday the Chinese balloon discovered this week over the United States was mainly for weather research. However, American officials said they have assessed it to be a collection device, though not one that could gather the kind of sensitive information that advanced Chinese reconnaissance satellites already collect.

China spends about $209 billion, or 1.3 percent 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.

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 the unidentified aerial phenomenon tracking, according to two U.S. officials.)

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.”

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

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 American 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 American sovereignty that highlighted the threat from Beijing.

Representative Mike Gallagher, Republican of Wisconsin, a member of the House Intelligence Committee and chairman of a new House committee on China, said the administration needs to tell lawmakers more about what it knows about surveillance of military facilities.

“This is all the more reason for the House Intelligence Committee to receive a full briefing on this matter,” Mr. 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 A. Milley, the chairman 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 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.

Helene Cooper and Adam Entous contributed reporting.