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.”