A U.S. fighter jet shot down an “unidentified object” over Lake Huron on Sunday on orders from President Joe Biden. It was the fourth such downing in eight days and the latest military strike in an extraordinary chain of events over U.S. airspace that Pentagon officials believe has no peacetime precedent.

The U.S. Air Force general overseeing North American airspace said on Sunday after a series of shoot-downs of unidentified objects that he would not rule out aliens or any other explanation yet, deferring to U.S. intelligence experts.

Asked whether he had ruled out an extraterrestrial origin for three airborne objects shot down by U.S. warplanes in as many days, General Glen VanHerck said: "I'll let the intel community and the counterintelligence community figure that out. I haven't ruled out anything."

"At this point we continue to assess every threat or potential threat, unknown, that approaches North America with an attempt to identify it," said VanHerck, head of U.S. North American Aerospace Defense Command and Northern Command.

VanHerck's comments came during a Pentagon briefing on Sunday after a U.S. F-16 fighter jet shot down an octagonal-shaped object over Lake Huron on the U.S.-Canada border.