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.

Part of the reason for the repeated shootdowns is a “heightened alert” following a spy balloon from China that emerged over U.S. airspace in late January, Gen. Glen VanHerck, head of NORAD and U.S. Northern Command, said in a briefing with reporters.

Since then, fighter jets last week also shot down objects over Canada and Alaska. Pentagon officials said they posed no security threats, but so little was known about them that Pentagon officials were ruling nothing out — not even UFOs.

“We have been more closely scrutinizing our airspace at these altitudes, including enhancing our radar, which may at least partly explain the increase,” said Melissa Dalton, assistant defense secretary for homeland defense.

U.S. authorities have made clear that they constantly monitor for unknown radar blips, and it is not unusual to shut down airspace as a precaution to evaluate them. But the unusually assertive response was raising questions about whether such use of force was warranted, particularly as administration officials said the objects were not of great national security concern and the downings were just out of caution.

VanHerck said the U.S. adjusted its radar so it could track slower objects. “With some adjustments, we’ve been able to get a better categorization of radar tracks now,” he said, “and that’s why I think you’re seeing these, plus there’s a heightened alert to look for this information.”

He added: “I believe this is the first time within United States or American airspace that NORAD or United States Northern Command has taken kinetic action against an airborne object.”

Asked if officials have ruled out extraterrestrials, VanHerck said, “I haven’t ruled out anything at this point.”

The Pentagon officials said they were still trying to determine what exactly the objects were and said they had considered using the jets’ guns instead of missiles, but it proved to be too difficult. They drew a strong distinction between the three shot down over this weekend and the balloon from China.

Minnesota Gov. Tim Walz tweeted that airmen in the 148th Fighter Wing, an Air National Guard fighter unit in Duluth, shot down the object over Lake Huron.

The extraordinary air defense activity began in late January, when a white orb the officials said was from China appeared over the U.S. and hovered above the nation for days before fighter jets downed it off the coast of Myrtle Beach, South Carolina. That event played out over livestream. Many Americans have been captivated by the drama playing out in the skies as fighter jets scramble to shoot down objects.

Objects Still Unidentified

The latest brought down was first detected on Saturday evening over Montana, but it was initially thought to be an anomaly. Radar picked it up again Sunday hovering over the Upper Peninsula of Michigan and it was going over Lake Huron, Pentagon officials said Sunday.

U.S. and Canadian authorities had restricted some airspace over the lake earlier Sunday as planes were scrambled to intercept and try to identify the object. According to a senior administration official, the object was octagonal, with strings hanging off, but had no discernable payload. It was flying low at about 20,000 feet, said the official who spoke to The Associated Press on condition of anonymity to discuss sensitive matters.

Meanwhile, U.S. officials were still trying to precisely identify two other objects shot down by F-22 fighter jets, and were working to determine whether China was responsible as concerns escalated about what Washington said was Beijing’s large-scale aerial surveillance program.

An object shot down Saturday over Canada’s Yukon was described by U.S. officials as a balloon significantly smaller than the balloon — the size of three school buses — hit by a missile Feb. 4. A flying object brought down over the remote northern coast of Alaska on Friday was more cylindrical and described as a type of airship.

Both were believed to have a payload, either attached or suspended from them, according to the officials who spoke to The Associated Press on condition of anonymity to discuss the ongoing investigation. Officials were not able to say who launched the objects and were seeking to figure out their origin.

The three objects were much smaller in size, different in appearance and flew at lower altitudes than the suspected spy balloon that fell into the Atlantic Ocean after the U.S. missile strike.

The officials said the other three objects were not consistent with the fleet of Chinese aerial surveillance balloons that targeted more than 40 countries, stretching back at least into the Trump administration.

Senate Majority Leader Chuck Schumer told ABC’s “This Week” that U.S. officials were working quickly to recover debris. Using shorthand to describe the objects as balloons, he said U.S military and intelligence officials were “focused like a laser” on gathering and accumulating the information, then compiling a comprehensive analysis.

“The bottom line is until a few months ago we didn’t know about these balloons,” Schumer, D-N.Y., said of the spy program that the administration has linked to the People’s Liberation Army, China’s military. “It is wild that we didn’t know.”

Eight days ago, F-22 jets downed the large white balloon that had wafted over the U.S. for days at an altitude of about 60,000 feet. U.S. officials immediately blamed China, saying the balloon was equipped to detect and collect intelligence signals and could maneuver itself. White House officials said improved surveillance capabilities helped detect it.

China’s Foreign Ministry said the unmanned balloon was a civilian meteorological airship that had blown off course. Beijing said the U.S. had “overreacted” by shooting it down.

Then, on Friday, North American Aerospace Defense Command, the combined U.S.-Canada organization that provides shared defense of airspace over the two nations, detected and shot down an object near sparsely populated Deadhorse, Alaska.

Later that evening, NORAD detected a second object, flying at a high altitude over Alaska, U.S. officials said. It crossed into Canadian airspace on Saturday and was over the Yukon, a remote territory, when it was ordered shot down by Prime Minister Justin Trudeau.

In both of those incidents, the objects were flying at roughly 40,000 feet. The object on Sunday was flying at 20,000 feet.

The cases have increased diplomatic tensions between the United States and China, raised questions about the extent of Beijing’s American surveillance, and prompted days of criticism from Republican lawmakers about the administration’s response.