The suspected Chinese surveillance balloon that violated American airspace this week has fueled a diplomatic crisis with the postponement of Secretary of State Antony Blinken's planned trip to Beijing.
But the two countries have a long history of spying on each other.
The US has sought to collect its own intelligence about the Chinese government, using methods that include flying surveillance aircraft over disputed islands claimed by Beijing, human sources and signal intercepts.
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Still, American officials have sought to distinguish US actions from what they say is the more brazen espionage being carried out by the Chinese government.
US officials say Beijing uses every tool at its disposal to gain a strategic advantage over the United States, its primary geopolitical rival. But Chinese officials say a similar thing -- Beijing has in the past repeatedly accused the US of espionage.
China denies that the balloon currently above the US is involved in any kind of espionage, claiming it is a "civilian airship used for research, mainly meteorological, purposes" that has been blown off course.
Here's what we know about how China spies on the US:
Balloons and satellites
While the suspected Chinese balloon spotted in the skies above multiple US states this week prompted an uproar from Republicans and Democrats alike, it is not the first time this kind of activity has been observed.
A US official said Friday there had been similar incidents over Hawaii and Guam in recent years, while another official on Thursday said, "Instances of this activity have been observed over the past several years, including prior to this administration."
US officials have said the flight path of the latest balloon, first spotted over Montana on Thursday, could potentially take it over a "number of sensitive sites." They say they are taking steps to "protect against foreign intelligence collection."
What's less clear is why Chinese spies would want to use a balloon, rather than a satellite to gather information.
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Using balloons as spy platforms goes back to the early days of the Cold War. Since then, the US has used hundreds of them to monitor its adversaries, said Peter Layton, a fellow at the Griffith Asia Institute in Australia and former Royal Australian Air Force officer.
But with the advent of modern satellite technology enabling the gathering of overflight intelligence data from space, the use of surveillance balloons had been going out of fashion.
Or at least until now.
Recent advances in the miniaturization of electronics mean the floating intelligence platforms may be making a comeback in the modern spying toolkit.
"Balloon payloads can now weigh less, and so the balloons can be smaller, cheaper and easier to launch" than satellites, Layton said.
Outside Malmstrom Air Force Base in central Montana, spread across 13,800 square miles of open plains, more than 100 intercontinental ballistic missiles stand at the ready, buried deep underground in missile silos. These Minuteman III rockets are capable of delivering nuclear warheads at least 6,000 miles away and are part of the US Strategic Command, which oversees the country's nuclear and missile arsenal.
Nestled among these silos are clusters of cell phone towers operated by a small rural wireless carrier. According to Federal Communications Commission filings, those cell towers use Chinese technology that security experts have warned in recent years could allow China to gather intelligence while also potentially mounting network attacks in the areas surrounding this and other sensitive military installations.
Huawei, the Chinese company that makes the tower technology, is shunned by the major US wireless carriers and the federal government over national security concerns.
Yet its technology is widely deployed by a number of small, federally subsidized wireless carriers that buy cheaper Chinese-made hardware to place atop their cell towers. In some cases, those cellular networks provide exclusive coverage to rural areas close to US military bases, CNN previously reported.
In 2018, the heads of major US intelligence agencies -- including the FBI and CIA -- warned Americans against using Huawei devices and products. Security experts say that having its technology deployed so close to the nation's arsenal of ICBMs could pose a far greater threat.
In 2017, the Chinese government offered to spend $100 million to buildan ornate Chinese garden at the National Arboretum in Washington, DC. Complete with temples, pavilions and a 70-foot white pagoda, the project thrilled local officials, who hoped it would attract thousands of tourists every year.
But when US counterintelligence officials began digging into the details, they found numerous red flags. The pagoda, they noted, would have been strategically placed on one of the highest points in Washington, just two miles from the US Capitol, a perfect spot for signals intelligence collection, multiple sources told CNN last year.
Chinese officials wanted to build the pagoda with materials shipped to the US in diplomatic pouches, which US Customs officials are barred from examining, the sources said.
Federal officials quietlykilledthe project before construction started.
The canceled garden is just one of the projects that has caught the eye of the FBI and other federal agencies during what US security officials say has been a dramatic escalation of Chinese espionage on US soil over the past decade.
Since 2017, federal officials have investigated Chinese land purchases near critical infrastructure, shut down a regional consulate believed by the US government to be a hotbed of Chinese spies and stonewalled what they saw as efforts to plant listening devices near sensitive military and government facilities.
Some of the things the FBI uncovered pertained to Chinese-made Huaweiequipment atop cell towers near US military bases in therural Midwest.
According to multiple sources, the FBI determined the equipment was capable of capturing and disrupting highly restricted Defense Department communications, including those used by US Strategic Command, which oversees the country's nuclear weapons.
Students, business people and scientists
CNN has also reported that Beijing has been leaning on expatriate Chinese scientists, businesspeople and even students in the US, according to current and former US intelligence officials, lawmakers and several experts.
There have been a number of high-profile arrests. In January, a former graduate student in Chicago was sentenced to eight years in prison for spying for the Chinese government by gathering information on engineers and scientists in the United States.
Ji Chaoqun, a Chinese national who came to the US to study electrical engineering at the Illinois Institute of Technology in 2013 and later enlisted in the US Army Reserves, was arrested in 2018.
The 31-year-old was convicted last September of acting illegally as an agent of China's Ministry of State Security (MSS) and of making a material false statement to the US Army.
According to the Justice Department, Ji was tasked with providing an intelligence officer with biographical information on individuals for potential recruitment as Chinese spies. The individuals included Chinese nationals who were working as engineers and scientists in the US, some of whom worked for American defense contractors.
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Ji's spying was part of an effort by Chinese intelligence to obtain access to advanced aerospace and satellite technologies being developed by US companies, the Justice Department said.
Ji was working at the direction of Xu Yanjun, a deputy division director at the Jiangsu provincial branch of the MMS, the DOJ statement said.
Xu, a career intelligence officer, was sentenced last year to 20 years in prison for plotting to steal trade secrets from several US aviation and aerospace companies. Xu was also the first Chinese spy extradited to the US for trial, after being detained in Belgium in 2018 following an FBI investigation.
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