The US military is unsure what three flying objects it shot out of the skies over North America were – and how they were able to stay aloft.
President Joe Biden ordered another object – the fourth in total this month – to be downed on Sunday.
As it was travelling at 20,000ft (6,100m), it could have interfered with commercial air traffic, the US said.
A military commander said it could be a “gaseous type of balloon” or “some type of a propulsion system”.
He added he could not rule out that the objects were extra-terrestrials.
The latest object – shot down over Lake Huron in Michigan near the Canadian border – has been described by defence officials as an unmanned “octagonal structure” with strings attached to it.
It was downed by a missile fired from an F-16 fighter jet at 14:42 local time (19:42 GMT).
The incident raises further questions about the spate of high-altitude objects that have been shot down over North America this month.
US Northern Command Commander General Glen VanHerck said that there was no indication of any threat.
“I'm not going to categorise them as balloons. We're calling them objects for a reason,” he said.
“What we are seeing is very, very small objects that produce a very, very low radar cross-section,” he added.
Speculation as to what the objects may be has intensified in recent days.
“I will let the intel community and the counterintelligence community figure that out,” Gen VanHerck said when asked if it was possible the objects are aliens or extra-terrestrials.
“I haven't ruled out anything at this point.”
A suspected Chinese spy balloon was downed off the coast of South Carolina on 4 February after hovering for days over the US. Officials said it originated in China and had been used to monitor sensitive sites.
China denied the object was used for spying and said it was a weather monitoring device that had blown astray. The incident – and the angry exchanges in its aftermath – ratcheted up tensions between Washington and Beijing.
But on Sunday, a defence official said the US had communicated with Beijing about the first object, after receiving no response for several days. It was not immediately clear what was discussed.
Since that first incident, American fighter jets have shot down three further high-altitude objects in as many days.
President Biden ordered an object to be shot down over northern Alaska on Friday, and on Saturday a similar object was shot down over the Yukon in north-western Canada.
Both the US and Canada are still working to recover the remnants, but the search in Alaska has been hampered by Arctic conditions.
“These objects did not closely resemble, and were much smaller than, the [4 February] balloon and we will not definitively characterise them until we can recover the debris,” a White House National Security spokesperson said.
China's foreign ministry said on Monday the US has flown balloons into its airspace more than 10 times in the past year.
“It's not uncommon as well for the US to illegally enter the airspace of other countries,” said foreign ministry spokesman Wang Wenbin at a press briefing.
Detection of the most recent objects could be a result of widening the search from radars and sensors.
“We have been more closely scrutinising our airspace at these altitudes, including enhancing our radar,” said Melissa Dalton, an assistant secretary of defence, said.
An official told the Washington Post it was like a car buyer unticking boxes on a website to broaden the parameters of what can be searched.
But he said it was unclear whether this was producing more hits – or if the new incursions are part of a more deliberate action.
Unidentified flying objects – timeline
4 February: US military shoots down suspected surveillance balloon off the coast of South Carolina. It had drifted for days over the US, and officials said it came from China and had been monitoring sensitive sites
10 February: US downs another object off northern Alaska which officials said lacked any system of propulsion or control
11 February: An American fighter jet shoots down a “high-altitude airborne object” over Canada's Yukon territory, about 100 miles (160 km) from the US border. It was described as cylindrical and smaller than the first balloon
12 February: US jets shoot down a fourth high-altitude object near Lake Huron “out of an abundance of caution”
One senior official told ABC News that the three most recent objects to be shot down were likely weather devices and not surveillance balloons.
But this was seemingly contradicted by the top Democrat in Congress, who earlier told the broadcaster that intelligence officials believed the objects were in fact surveillance balloons.
“They believe they were [balloons], yes,” Senate Majority Leader Chuck Schumer said, adding that they were “much smaller” than the first one shot down off the South Carolina coast.
Democratic Senator Jon Tester, who represents Montana, told the BBC's US partner CBS: “What's gone on the last two weeks or so… has been nothing short of craziness.”
Republicans have repeatedly criticised the Biden administration for its handling of the first suspected spy balloon, saying it should have been shot down far sooner.
Other countries are watching the response in the US closely, in case an object is discovered in their airspace.
In the UK, Prime Minister Rishi Sunak said his government would do “whatever it takes” to keep the country safe.
“We have something called the quick reaction alert force which involves Typhoon planes, which are kept on 24/7 readiness to police our airspace, which is incredibly important,” he added.