US military officials say they are not sure how three unidentified flying objects shot down from the skies over North America managed to stay aloft.
President Joe Biden ordered another protest — the fourth this month — to be taken down on Sunday.
The US said that because it was traveling at an altitude of 20,000 feet (6,100m), it could have interfered with commercial air traffic.
A military commander said it could be a “gas type balloon” or “some kind of propulsion system”.
He added that he could not rule out that these objects were extraterrestrial.
The latest object – shot down near the Canadian border – was described by defense officials as an unmanned “octagonal structure” with wires attached to it.
It was shot down by a missile fired from an F-16 fighter jet at 14:42 local time (19:42 GMT).
The incident has raised more questions about high-altitude objects in North America this month.
General Glenn Van Hurk, commander of the US Northern Command, said there was no indication of a threat.
“I’m not going to classify them as balloons. We’re calling them objects for a reason,” he said.
“What we’re seeing are very, very small objects that produce a very, very low radar cross section,” he added.
Speculation has intensified in recent days as to what these objects might be.
“I’ll let the intelligence community and the counterintelligence community figure that out,” General Van Hurk said when asked if it was possible the objects were alien or extraterrestrial.
“I haven’t ruled anything out at this point.”
A suspected Chinese spy balloon was shot down off the coast of South Carolina on February 4 after hovering over the US for several days. Officials say it originated in China and was used to monitor sensitive locations.
China denied using the object for espionage and said it was a weather monitoring device that had misled. The incident and the angry exchanges that followed heightened tensions between Washington and Beijing.
But a defense official said on Sunday that the US had first spoken to Beijing about the objection after days without a response. It was not immediately clear what the conversation was.
Since that first incident, US fighter jets have downed three more high-altitude objects in as many days.
President Biden ordered an object shot down over northern Alaska on Friday, and a similar object was shot down over the Yukon in northwestern Canada on Saturday.
Both the United States and Canada are still working to recover the remains, but the search is hampered by arctic conditions in Alaska.
A White House national security spokesman said, “These objects did not closely resemble, and were much smaller than, the [Feb. 4] balloon and until we remove the debris, we will not characterize them definitively. Will do.”
