WASHINGTON: On orders from President Joe Biden, a U.S. fighter jet downed an "unidentified object" over Lake Huron on Sunday. It was the latest military strike in an extraordinary string of events over American airspace that, according to Pentagon officials, has no precedent during peacetime. It was the fourth such downing in eight days.

According to Gen. Glen VanHerck, head of NORAD and U.S. Northern Command, part of the reason for the repeated shootdowns is a "heightened alert" following a spy balloon from China that appeared over U.S. airspace in late January.

Since then, fighter jets have also destroyed objects over Alaska and Canada last week. However, because so little was known about them, Pentagon officials were not ruling anything out, not even UFOs, when they claimed they posed no security threats.

According to Melissa Dalton, assistant defense secretary for homeland defense, "We have been more closely scrutinizing our airspace at these altitudes, including enhancing our radar, which may at least partially explain the increase."

VanHerck responded, "I haven't ruled anything out at this point," when asked if officials had ruled out extraterrestrial life. Officials from the Pentagon stated that they were still trying to identify the precise nature of the objects and that they had thought about using the jets' guns rather than missiles, but the task proved to be too challenging.

They made a clear distinction between the balloon from China and the three that were shot down over the weekend. Tim Walz, the governor of Minnesota, tweeted that the object over Lake Huron was shot down by members of the 148th Fighter Wing, an Air National Guard fighter unit in Duluth.

The excellent air defense activity began in late January, when a white orb that 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. Many Americans have been captivated by the drama playing out in the skies as fighter jets scramble to shoot down objects.

U.S. officials claimed that an object shot down over Canada's Yukon on Saturday was a balloon that was considerably smaller than the balloon that was struck by a missile on February 4 and was the size of three school buses. Officials were trying to determine the origin of the objects but were unable to determine who launched them. The three objects flew at lower altitudes and were significantly smaller in size and appearance than the alleged spy balloon that crashed into the Atlantic Ocean following the American missile strike.

The other three objects, according to the officials, were inconsistent with the fleet of Chinese aerial surveillance balloons that had been tracking more than 40 nations at least as far back as the Trump administration. He said U.S. military and intelligence officials were "focused like a laser" on gathering and accumulating the information before compiling a thorough analysis. He used shorthand to describe the objects as "balloons." The spy program, which the administration has linked to China's military's People's Liberation Army, was unknown to the United States until a few months ago, according to Schumer, a Democrat from New York.

Eight days ago, an enormous white balloon that had been circling the United States for days at a height of about 60,000 feet was shot down by F-22 jets. U.S. officials immediately placed the blame on China, claiming that the balloon could self-maneuver and was equipped to detect and gather intelligence signals. Improved surveillance capabilities, according to White House officials, helped identify it.

The unmanned balloon was a civilian meteorological airship that had veered off course, according to China's Foreign Ministry. Then, on Friday, an object was discovered and shot down close to the sparsely populated Deadhorse, Alaska, by the joint U.S.-Canada organization known as North American Aerospace Defense Command. Later that night, over Alaska, NORAD discovered a second object flying at a great height, according to American officials.

The objects in both of those incidents were traveling at a height of about 40,000 feet. On Sunday, the object was traveling at 20,000 feet.

The cases have heightened diplomatic tensions between the United States and China, sparked concerns about the scope of American surveillance of Beijing, and led to days of criticism of the administration's response from Republican lawmakers.