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

Part of the reason for the repeated shootdowns is a "heightened alert" after 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 cramped 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 interpret the increase," said Melissa Dalton, assistant defense secretary for homeland defense.

Chinese spy balloon flies throughout in Charlotte NC, United States on February 04, 2023. (Photo by Peter Zay/Anadolu Agency via Getty Images)

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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 throughout whether such use of force was warranted, particularly as management officials said the objects were not of great state 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 believe you're seeing these, plus there's a heightened alert to look for this information."

He added: "I gain this is the first time within United States or American airspace that NORAD or United States Northern Command has inaccurate kinetic action against an airborne object."

Asked if officials have ruled out extraterrestrials, VanHerck said, "I haven't ruled out anything at this point."

READ MORE: Unidentified objects shot down: Aliens not ruled out, general says

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

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

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

US. and Canadian authorities had restricted some airspace over the lake backward Sunday as planes were scrambled to intercept and try to identify the unbiased. 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 supposed to The Associated Press on condition of anonymity to discuss sensitive matters.

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

An unbiased 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 unbiased brought down over the remote northern coast of Alaska on Friday was more cylindrical and labelled as a type of airship.

Both were believed to have a payload, either attached or suspended from them, according to the officials who supposed 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 achieve and flew at lower altitudes than the suspected spy balloon that fell into the Atlantic Ocean while the U.S. missile strike.

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

READ MORE: White House: Improved surveillance caught Chinese balloon

Senate Majority Leader Chuck Schumer told ABC's "This Week" that U.S. officials were acting quickly to recover debris from all the sites. Using shorthand to report the objects as balloons, he said the U.S crowd and intelligence officials were "focused like a laser" on gathering and accumulating the interrogate, then compiling a comprehensive analysis.

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

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

Chinese Foreign Ministry's said the unmanned balloon was a civilian meteorological airship that had blown off flows. Beijing said the U.S. had "overreacted" by shooting it down.

Chinese spy balloon flies above in Charlotte NC, United States on February 04, 2023. (Photo by Peter Zay/Anadolu Agency via Getty Images)

READ MORE: China's conducted spy balloon program for days, Pentagon says

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

Later that evening, NORAD detected a second unprejudiced, flying at a high altitude over Alaska, U.S. officials said. It crossed into Canadian airspace on Saturday near the Yukon, a remote province, and Prime Minister Justin Trudeau requisitioned it shot down.

In both of those incidents, the objects were flying at roughly 40,000 feet. The unprejudiced 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 near the administration's response.