First came the Chinese spy balloon off the coast of South Carolina.
Six days later, it was another flying object over sea ice in Alaska.
Then came the "small cylindrical object" in Yukon, northern Canada, and an "octagonal object" over Lake Huron on the border.
These four shoot-downs within eight days are the first in the history of the US-Canadian North American Aerospace Defense Command, originally set up during the early Cold War.
The flurry of aerial activity has prompted a wave of questions — and a sprinkling of suspicion — about what the objects were, where they came from, and what made them dangerous enough for US fighter jets to shoot them out of the sky.
US officials have revealed the balloon shot down near Myrtle Beach on February 4 was part of a broader Chinese surveillance program connected to the People's Liberation Army.
But less is known about the subsequent three objects.
And with authorities yet to retrieve the debris and begin to answer some of these questions, Congress is pushing for greater transparency around the unidentified aerial phenomena (UAP).
Meanwhile, analysts in the intelligence community are wrapping their heads around why a modern geopolitical superpower is resorting to a relatively low-tech surveillance tool, and what that could mean for the future.
Why is the US shooting these things down?
In the case of the balloon, US officials are confident it was "seeking to monitor sensitive military sites", despite China's initial insistence that it was a civilian research craft blown off course.
US analysts are examining if wild weather did alter the balloon’s intended flight path, considering the possibility China may have only been seeking to capitalise on an accidental incursion into the American heartland.
The Washington Post recently revealed US military and intelligence agencies tracked the balloon for nearly a week after it was launched from Hainan Island, near China’s south coast.
And the Pentagon confirmed last week that the agencies had been monitoring the spy balloon program for several years, with the fleet also spotted over Latin America, South America, parts of Asia, and Europe.
"The concern is that … China would be collecting on our sensitive, advanced military capabilities, how we train, the techniques we use," explained Kari Bingen, aerospace director at the Center for Strategic and International Studies and a former senior defence official.
"That erodes our military and technology advantage."
Ms Bingen says the spy balloon program is an interesting development given China's other surveillance capabilities.
"Nations spy on each other … but why balloon when they have a pretty robust satellite, or space program? China has doubled the number of satellites they placed on orbit in the last couple of years," she said.
She explains there are benefits to balloons — they can get closer to target sites than satellites can, are often able to hover, and allow for less predictable paths.
The cost is lower, and there is less technical complexity to build them. But they move slowly, and are more conspicuous than most satellites.
"At 200 feet tall, that's like a 20-storey building. So that's hard to miss."
The three other as-yet-unidentified objects were "very different" from the Chinese spy balloon, according to US officials.
General Glen VanHerck, who oversees North American airspace, told a briefing on Sunday that these objects "do not pose a kinetic military threat, and I don't see that changing even when we recover the debris".
Ultimately, it was the route they took — over other sensitive sites — and the altitude of these objects that caused concern.
While there is no international agreement that specifies the vertical boundary of a nation's airspace, the International Civil Aviation Organization states "each state has complete and exclusive sovereignty over the airspace above its territory".
That includes the airspace over a country's land mass, and extends across territorial seas, 12 nautical miles out (which is around 22.2km or 72,800 feet).
Peter Layton, a military analyst and visiting fellow at Griffith University's Asia Institute argues that "if a balloon overflies a country below 22.2 kilometres altitude, that country's sovereignty has been infringed and the craft can be legally shot down".
According to the Federal Aviation Administration, the airspace over US territory from 18,000 feet up to 60,000 feet is considered Class A controlled airspace, which requires air traffic control clearance.
US officials have said the objects spotted in Alaska and Yukon were flying at around 40,000 feet — towards the upper limit of most commercial passenger jets' cruising altitude — while the object near Lake Huron was much lower at 20,000 feet.
They said all three objects were shot down because they posed a risk to civilian aircraft.
Balloons, birds or aliens — what could these unidentified objects be?
Currently, there is no indication the unidentified objects were part of China's spy balloon program or another foreign surveillance program.
The leading explanation put forward by the White House and other officials appears to be that the devices were launched without sinister intent.
"One thing we have to consider, and we believe the intelligence community is considering as an explanation, is that these could be balloons tied to commercial or research entities and therefore totally benign," Pentagon press secretary John Kirby said on Tuesday.
Officials say the objects — which were much smaller than the balloon — were similar to each other in size and shape, but investigators need to get their hands on the debris to understand how they work.
At this stage, they're not calling them balloons.
"We call them objects for a reason … I am not able to categorise how they stay aloft. It could be a gaseous type of balloon inside a structure or it could be some type of a propulsion system," General VanHerck said on Sunday.
It's not the first time this type of aerial activity has caught the attention of US intelligence.
"Sometimes it's attributed to potentially being birds, sometimes it's been attributed to weather. Sometimes we don't know what to attribute it to," General VanHerck said.
One scenario the White House has poured cold water on is extraterrestrial involvement.
"I know there have been questions and concerns about this, but there is, again, no indication of aliens or extraterrestrial activity with these recent take-downs," White House press secretary Karine Jean-Pierre said on Monday.
General VanHerck had earlier told reporters asking about the possibility: "I'll let the counterintelligence community figure that out — I haven't ruled out anything."
But several other national security officials have dismissed this idea, with one telling the New York Times "no-one thinks these things are anything other than devices fashioned here on Earth".
That hasn't stopped a small but passionate group of Americans from sharing their own theories about the real origins of these objects.
A patchy history of transparency and distrust around UFOs
The US intelligence community has been monitoring unidentified flying objects (now known as unidentified aerial phenomena) for decades — though until recently, this was mostly done within a cone of secrecy.
Many academics pinpoint the Roswell incident of 1947 as the beginning of Americans' suspicion relating to mysterious flying objects.
When a top-secret US military balloon crashed in the desert near Roswell, New Mexico, the army initially reported that they had found "a flying disc", before quickly changing the statement to describe it as a "weather balloon".
Decades later, a report revealed the true capabilities of the balloon, but increasingly complex theories involving alien bodies and flying saucers persist to this day.
A third of Americans still believe, 75 years later, that it's plausible that extraterrestrials crash-landed at Roswell, with 12 per cent saying that it is “very likely to be true".
This historical distrust has at times given rise to conspiracy groups, religious groups and dangerous cults, including the infamous Heaven's Gate, which led 39 followers to mass suicide in pursuit of immortal extraterrestrial bodies.
But separate to these extreme religious elements, members of the so-called ufology community have long been lobbying the US government to publish information about UFOs.
In the early 1990s, an emergency room doctor named Steven M Greer founded the Disclosure Project, a group that aimed to publicly disclose the government's alleged knowledge of UFOs.
In 2001, he addressed a National Press Club event in Washington, DC, and released an almost-500-page dossier detailing what he described as the US government's efforts to conceal the truth.
Greer's movement had hoped their push for greater transparency would bring the issue to the attention of Congress and gain momentum, but it remained a relatively fringe issue for a while.
In 2017, two New York Times reporters teamed up with independent journalist and UFO researcher Leslie Kean on an expose revealing a long-running UFO program run by the Pentagon.
This was seen as a turning point by many in the ufology community, shifting the public discourse about UAPs.
Members of Congress began to talk more openly about US Air Force and Navy videos that showed objects no-one could explain, and in 2020, the Pentagon set up a new body to investigate these reports.
The Office of the Director of National Intelligence (ODNI) delivered its preliminary UAP report in June 2021, including 144 incidents that occurred between 2004 and March 2021 — only one of which was able to be explained.
Last year, Congress held the first hearings on unidentified aerial objects in about 50 years, and President Joe Biden signed off on a bill requiring the Defense Department to review historical UAP documents back to 1945.
About 18 months after its preliminary report, the ODNI's first annual UAP report summarised 247 new events logged after March 2021, and 119 historical events that hadn't been included previously.
More than half were characterised as unremarkable: 26 that were likely unmanned aircraft systems (drones), 163 balloon or balloon-like entities, and six attributed to clutter.
But 171 UAP incidents required further analysis and remain unidentified.
Along with the government's shift in transparency around UAPs, the public sentiment on unidentified aerial phenomena is also changing.
According to a 2021 Gallup poll, 41 per cent of Americans believe that at least some unidentified flying objects are alien spacecraft. That's up from 33 per cent in the same poll delivered in 2019.
In 2021, Pew Research Center found 51 per cent of Americans believe UFOs reported by people in the military are definitely or probably evidence of intelligent life outside Earth, but most don't see UFOs as a major security threat.
Even Greer, seen as "the father of the Disclosure movement", doesn't think these three unidentified objects are extraterrestrial devices.
But he has speculated that the national discussion around this possibility is a convenient way to distract the public from evidence he says the US government does have of other extraterrestrial activity on Earth.
It's not only members of the public who are pushing for more transparency on these issues.
After a briefing this week, US senators have called on the president to speak directly with the public to allay their concerns about the latest objects.
"I am not in any way afraid that we are under a threat of attack or physical harm to our homeland. That's my personal feeling. But the American people need to be reassured with more facts," Democratic senator for Connecticut Richard Blumenthal said on Tuesday.
If not new, why do four appear in a week? And will there be more?
Clearly, mysterious sky-dwellers are nothing new — high-altitude balloons have been used for surveillance since at least the Cold War, and are regularly deployed for scientific research and weather monitoring.
A senior defence official told reporters on February 4 that Chinese surveillance balloons had transited the continental United States "at least three times during the prior administration and once that we know of at the beginning of this administration".
China's foreign ministry has also claimed that the US sent spy balloons into its airspace 10 times since last year.
As for other unidentified aerial phenomena, the ODNI concluded that the increase in UAP reports during 2022 was likely down to a combination of two things: a better understanding of possible threats, and reduced stigma surrounding UAP reporting.
In the case of these particular unidentified objects, defence officials have said the US stepped up the scrutiny of its airspace after the balloon incident, "which may at least partly explain the increase in objects that we've detected over the past week".
After the balloon was detected, the North American Aerospace Defense Command (NORAD) adjusted its radar system to make it more sensitive.
"[Most radars] have a velocity gate that says, 'OK, show me everything that's travelling faster than, say, 500kph'," Dr Layton explained.
"As you wind that velocity gate back to, say, 100kph, it will show you all the cars on the road, as well as the airplanes.
"If you wind it right back, balloons travel at the speed of the wind, which is the speed the clouds do.
"So that's why all of the radars normally will not pick up something like this — they're not looking for very slow-moving objects. There's too much clutter."
Dr Layton said the objects detected after the balloon incident were likely picked up thanks to a combination of fine-tuning the radar and using electronic signature data from the US intelligence community.
It's likely US intelligence will continue to adjust its approach to detecting unknown objects, and General VanHerck told reporters on Sunday: "If it is a threat, I'll shoot it down."
Ms Bingen says while it remains to be seen what sorts of devices the three other objects could be, these events are important to consider in the broader context of increasingly aggressive tactics being deployed by China.
"I do think that these events should serve as a wake-up call to the American people that China can reach the American homeland," she said.
"The risk that China poses is not some distant fight across the Pacific."
Dr Layton is also wary of the recent shift in tactics from Beijing, but notes: "Global wind patterns mean [spy balloon] operations are likely to be restricted to the northern hemisphere unless the balloons are launched off Chinese naval ships."
So, could the US or its allies revisit the era of surveillance balloons in future?
"I would just say, from my experience, we have really good intelligence capabilities," Ms Bingen said.
