A U.S. fighter jet shot down a high-flying object above Lake Huron Sunday after it flew near a sensitive military site, the Pentagon said.
The shoot-down, which came on President Joe Biden’s order, is the latest in an unusual series of aerial incidents that began last week when a suspected Chinese spy balloon traversed the continental United States.
At 2:42 p.m. Eastern time, an F-16 launched a heat-seeking AIM-9X missile at an unidentified “object” flying at around 20,000 ft. above the Great Lake. It was the fourth time in eight days that the American military has been called upon to destroy a mysterious craft in midair, including three in the last three days.
Other than the first balloon, which China has acknowledged was theirs, the Pentagon doesn’t yet know what the objects are or where they came from. Asked directly, a senior military official did not rule out the possibility that their provenance could be extraterrestrial.
“I’ll let the intel community and the counterintelligence community figure that out,” General Glen VanHerck, commander of U.S. Northern Command and the North American Aerospace Defense Command (NORAD) told reporters late Sunday.
“I haven’t ruled out anything,” VanHerck said when asked if aliens could be involved. “At this point, we continue to assess every threat or potential threat unknown that approaches North America with an attempt to identify it.”
Read More: Chinese Balloon Wreckage Could Offer Clues Into Years-Long Spy Operation
The Pentagon is now trying to obtain debris from each of the four shoot-downs in an effort to learn more about the crafts penetrating North American airspace. VanHerck acknowledged the military is perplexed.
“I’m not going to categorize them as balloons,” VanHerck said. “We’re calling them objects for a reason. I’m not able to categorize how they stay aloft.” None of the flying objects have posed a threat to anything on the ground, VanHerck said, but they have been assessed to be safety hazards to civilian air traffic and potential surveillance threats.
The flurry of incidents is due at least in part to an increase in military vigilance in the wake of the Chinese balloon. NORAD has tweaked its radar “gates” to account for lower speeds and sizes to spot more flying objects. “We have adjusted some of those gates to give us better fidelity on seeing smaller objects,” VanHerck said. “You can also filter out by altitude. And so, with some adjustments, we’ve been able to get a better categorization of radar-tracks now.”
Read More: How an Alleged Spy Balloon Derailed an Important U.S.-China Meeting
Every day, NORAD monitors airspace across North America. On Saturday at about 4:45 p.m. Eastern Time, the military command registered a radar contact in Canadian airspace, about 70 miles north of the U.S. border. The craft was not in communication with the Federal Aviation Administration, so VanHerck sent F-15 fighters from Portland, Oregon, to investigate over Montana. The F-15s arrived at 7:04, and within 45 minutes it became dark. The aviators ultimately couldn’t identify and locate the object in the dark. VanHerck didn’t identify the “sensitive” military site that the object flew over, but Montana is home to a sprawling field where nuclear-armed intercontinental ballistic missiles sit ready-for-launch in underground silos. The Chinese balloon lingered over that site, according to U.S. officials.
On Sunday morning, NORAD made another radar contact near Wisconsin. “It’s likely, but we have not confirmed that the track that we saw in Wisconsin was likely the same track in Montana,” VanHerck said. NORAD tracked the object over Lake Michigan, he said, and as it passed over the state’s upper peninsula, the military engaged it.
Up until a week ago, the U.S. had never shot down an airborne object in North American airspace that was deemed to be a threat. Now the extraordinary has come to seem commonplace. On Feb. 4, the U.S. blasted apart the mammoth Chinese balloon over the Atlantic Ocean. On Feb. 10, on Friday, an American fighter jet shot down a small car-sized “object” off the Alaskan coast. The next day a fighter brought down another “object” over northern Canada.
The Pentagon and the Director of National Intelligence has an ongoing review of hundreds of UFO incidents documented in recent years without explanation. Melissa Dalton, assistant secretary of defense for homeland defense and hemispheric affairs, said the Biden Administration hasn’t made a policy change in how it treats unidentified flying objects in U.S. airspace.
“We are taking this pretty much on a case-by-case basis,” she told reporters during the briefing on Sunday. “Each operation has been different. And we will certainly keep you updated as we continue to learn more about these objects and the (People’s Republic of China’s) balloon and what that means for us going forward.”
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Write to W.J. Hennigan at william.hennigan@time.com