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.”