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.
US President Joe Biden ordered another object – the fourth in total this month – to be downed on Sunday.
As it was travelling at 20,000ft, 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.
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