WASHINGTON: U.S. military fighter jets on Sunday shot down an octagonal object over Lake Huron, the Pentagon said, the latest incident since a suspected Chinese surveillance balloon put North American security forces on high alert. It was the fourth unidentified flying object to be shot down over North America by a U.S. missile in a little more than a week. On President Joe Biden’s order, a U.S. F-16 fighter shot down the object at 2:42 p.m. local time over Lake Huron on the U.S.-Canada border, Pentagon spokesperson Patrick Ryder said in an official statement.

Though it did not pose a military threat, the object could have potentially interfered with domestic air traffic as it was traveling at 20,000 feet (6,100 m), and it might have had surveillance capabilities, Ryder said.

The object appeared to be octagonal in structure, with strings hanging off but no discernable payload, said a U.S. official speaking on condition of anonymity.

The object was recently detected over Montana near sensitive military sites, prompting the closure of U.S. airspace, the Pentagon said.

The incident raised questions about the spate of unusual objects that have appeared over North American skies in recent weeks and raised tensions with China.

“We need the facts about where they are originating from, what their purpose is, and why their frequency is increasing,” said U.S. Representative Debbie Dingell, one of several Michigan lawmakers who applauded the military for downing the object.

U.S. officials identified the first object as a Chinese surveillance balloon and shot it down off the coast of South Carolina on February 4. On Friday, a second object was shot down over sea ice near Deadhorse, Alaska. And a third object was destroyed over Canada’s Yukon on Saturday, with investigators still hunting for the wreckage.

“The security of citizens is our top priority and that’s why I made the decision to have that unidentified object shot down,” Prime Minister Justin Trudeau told reporters on Sunday.

North America has been on guard against aerial intrusions following the appearance of the white, eye-catching Chinese airship over American skies earlier this month.

That 200-foot-tall (60-meter-high) balloon – which Americans have accused Beijing of using to spy on the United States – caused an international incident, leading Secretary of State Antony Blinken to call off a planned trip to China only hours before he was set to depart.

Pentagon officials said they have been scrutinizing radar more closely since then and have not been able to identify what the most recent objects are, or how long they are staying aloft.

North America has been on guard against aerial intrusions following the appearance of the white, eye-catching Chinese airship over American skies earlier this month.

That 200-foot-tall (60-meter-high) balloon – which Americans have accused Beijing of using to spy on the United States – caused an international incident, leading Secretary of State Antony Blinken to call off a planned trip to China only hours before he was set to depart.

Pentagon officials said they have been scrutinizing radar more closely since then and have not been able to identify what the most recent objects are, or how long they are staying aloft.

“We’re calling them objects, not balloons, for a reason,” said Air Force General Glen VanHerck, who safeguards U.S. airspace as head of North American Aerospace Defense Command (NORAD) and Northern Command, told reporters.

The military will try to recover the object downed over Lake Huron, which they said likely fell in Canadian waters, to learn more about it, VanHerck told reporters.

Surveillance fears appear to have U.S. officials on high alert.

Twice in 24 hours, U.S. officials closed airspace – only to reopen it swiftly.

On Sunday, the Federal Aviation Administration briefly closed space above Lake Michigan. On Saturday, the U.S. military scrambled fighter jets in Montana to investigate a radar anomaly there.

China denies the first balloon was being used for surveillance and says it was a civilian research craft. It condemned the United States for shooting it down off the coast of South Carolina last Saturday.

U.S. Senate Majority Leader Chuck Schumer told U.S. broadcaster ABC that U.S. officials think two of the latest objects were smaller balloons than the original one.

The White House said only that the recently downed objects “did not closely resemble” the Chinese balloon, echoing Schumer’s description of them as “much smaller.”

“We will not definitively characterize them until we can recover the debris, which we are working on,” a spokesperson said.