Officials from the US military say they don't know how three unidentified flying objects that shot out of North American skies managed to stay in the air.
The fourth object to be brought down this month was ordered on Sunday by President Joe Biden.
The US claimed that due to its altitude of 20,000 feet (6,100 meters), the object might have impeded commercial air traffic.
It might be a "gaseous type of balloon" or "some kind of propulsion system," a military commander speculated.
The possibility of extraterrestrial life being present, he added, could not be ruled out.
According to defense officials, the most recent target, which was shot down close to the Canadian border, was an unmanned "octagonal structure" with strings attached.
At 14:42 local time (19:42 GMT), a fighter jet firing a missile downed the aircraft.
The incident raises more concerns about the recent spate of high-altitude objects shot down over North America.
There are no signs of a threat, according to US Northern Command Commander General Glen VanHerck.
"I'm not going to categorise them as balloons. They are called objects for a reason, he continued.
"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.
If the objects are extraterrestrials or aliens, Gen. VanHerck said, "I will let the intelligence community and the counterintelligence community figure that out.".
I haven't ruled anything out as of yet. ".
On February 4, a suspected Chinese spy balloon that had been circling the US for days was shot down off the coast of South Carolina. It was used to keep an eye on sensitive sites, according to officials, and it was Chinese in origin.
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.
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.
Democrat Debbie Dingell, one of several Michigan members of Congress who applauded the military for downing the object over the state on Sunday, joined growing calls for the White House and defence officials to provide more information.
"We need the facts about where they are originating from, what their purpose is, and why their frequency is increasing," she said.
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. And the military needs to have a plan to not only determine what's out there, but determine the dangers. ".
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.
Meanwhile, Defence Secretary Ben Wallace said the UK would conduct a security review following the recent incidents in the US and Canada.
"This development is another sign of how the global threat picture is changing for the worse," he said.
