The US military has shot down the fourth unidentified object to fly over US or Canadian skies this month.
A Chinese balloon that floated across the United States sparked the first interception on February 4.
Since then, objects have been shot down in Alaska, Yukon and Ontario.
So why are we suddenly hearing about unidentified objects, and what are they?
What were each of the objects shot down?
The United States is still assessing the debris, but officials have released some details about the objects.
The first:
- A large balloon shot down on February 4
- The US said it was equipped to detect and collect intelligence signals
- It was the size of about three school buses
- The US military remains tight-lipped about what it has learned from the recovery of the debris
The second:
- The object was shot down on February 10
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It came down over sea ice near Deadhorse in Alaska
The third:
- This one was shot down in central Yukon in Canada
- Canadian Defence Minister Anita Anand said it was a "small cylindrical object"
- The military determined what it was but would not reveal details, a North American Aerospace Defense Command (NORAD) spokesman said
The fourth:
- The object was shot down on Sunday afternoon local time
- It came down at Lake Huron, near the United States-Canadian border
- A US official said it appeared to be octagonal in structure, with strings hanging off it
What are the objects being used for?
China said the first balloon was for civilian meteorological and scientific purposes and it had strayed into US airspace.
But US President Joe Biden's administration described it as a manoeuvrable "surveillance balloon".
US and Canada officials are yet to publicly determine what the other three objects were being used for, or where they came from.
Ms Anand refused to speculate whether the object shot down over Yukon came from China.
"We are continuing to do the analysis on the object and we will make sure that analysis is thorough," she said on the weekend.
"It would not be prudent for me to speculate on the origins of the object at this time."
Are the recent objects related to the Chinese spy balloon?
China's foreign ministry said it did not have any information about the flying objects that were shot down by the US over Alaska and Canada.
A spokesperson for the White House National Security Council on Sunday said the three objects weren't as big as the balloon on February 4.
"These objects did not closely resemble and were much smaller than the PRC (People's Republic of China) balloon, and we will not definitively [characterise] them until we can recover the debris, which we are working on," the spokesperson said.
The statement came in response to queries about Senate Majority Leader Chuck Schumer's comment that US officials believed the unmanned objects were balloons.
Iain Boyd, from the University of Colorado, said there was "quite a lot of doubt" about whether all four objects were from China.
He said they could be weather or science balloons and the armed forces felt "it was necessary to not take any chances and [to] take them down".
"There has been a lot of political pressure to address these unknown objects more rapidly than the first case," Professor Boyd said.
How were they shot down?
The US has used fighter jets with missiles to shoot the objects from the sky.
Footage from the February 4 operation shows the missile hitting the Chinese balloon, releasing a white cloud of air.
Why are there suddenly multiple reports of flying objects?
The US is on "heightened alert" after the spy balloon incident.
That's according to General Glen VanHerck, Commander for NORAD and United States Northern Command.
He said the US adjusted its radar so it could track slower objects.
"With some adjustments, we've been able to get a better [categorisation] of radar tracks now," he said.
"And that's why I think you're seeing these, plus there's a heightened alert to look for this information."
He believed it was the first time within United States or American airspace that NORAD or United States Northern Command "had taken kinetic action against an airborne object".
United States Assistant Secretary of Defense for Homeland Defense and Global Security Melissa Dalton echoed that the US had become more vigilant.
"We have been more closely scrutinising our airspace at these altitudes, including enhancing our radar, which may at least partly explain the increase," she said.
Chinese foreign ministry spokesperson, Wang Wenbin, told reporters on Monday that US high-altitude balloons had flown over its airspace without permission more than 10 times since the beginning of 2022.
Could aliens be behind the unidentified objects?
Asked if officials had ruled out extraterrestrials, General VanHerck said: "I haven't ruled out anything at this point."
He said authorities were not referring to the latest objects as balloons because they were yet to determine how they were being kept aloft.
Another official, who spoke on the condition of anonymity, said the military had seen no evidence suggesting that any of the objects in question were of extraterrestrial origin.
