One reason why additional “objects” have been detected by the North American Aerospace Defense Command (NORAD) in recent days could be because the command recently readjusted its filters to better spot slow-moving targets operating above a certain altitude, a source briefed on the matter told CNN.  

The filters were only readjusted and broadened in the past week, the source said, after a high-altitude, suspected Chinese spy balloon transited the US and ignited a debate over the United States' ability to detect and defend against any potentially threatening objects entering its airspace. 

The Washington Post first reported on NORAD’s adjustment.

The US was able to track the spy balloon’s path before it entered US airspace -- and ultimately shoot it down off the coast of South Carolina -- in large part because the US intelligence community developed a method within the past year to track the balloons using a particular set of signals they emit, as CNN has previously reported.

But, in general, NORAD has tended to prioritize the detection of fast-moving targets below a certain altitude -- at which threatening planes or certain missiles, for example, might fly.

The more narrow filters were meant to allow NORAD and defense officials to better make sense of the mass of data that was being collected on any particular day, the source said. If they didn’t filter out slow-moving objects, early warning air defense systems would pick up lots of noise, such as weather balloons and birds. 

Two of the three objects shot down in the past three days-- near Alaska and over northern Canada -- were flying at around 40,000 feet, US and Canadian officials said, posing a potential risk to civilian aircraft. Both of the objects also appeared to feature a balloon, with a small metal cylinder underneath, officials said.

Very little is known so far about the third object shot down near Lake Huron on Sunday.