"With the cooperation of the Federal Aviation Administration, North American Aerospace Defense Command (NORAD) implemented a temporary flight restriction airspace over Lake Michigan… to ensure the safety of air traffic in the area during NORAD operations," the release read.
The flight restriction has since been lifted, NORAD said, without specifying details of its operations.
On Saturday, the United States closed the airspace around Havre, in the state of Montana near the border with Canada, saying that the closure was implemented to support US Department of Defense activities. US Senator from Montana Jon Tester said on Twitter that he was "aware of the object in Montana air space." No suspicious objects were eventually detected over Montana and the restriction was scrapped.
The US and Canada shot down a total of three high-altitude airborne objects in February, including one that was taken down on Saturday over Canada's Yukon Territory.