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, a US official told the Washington Post on Saturday.
The official, who requested anonymity, likened it to a car buyer unchecking 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.