What we know about the flying objects over North America

One senator even described the past few weeks as "nothing short of craziness".

But there are many unanswered questions. So here's what we know - and what we still don't.

How it all began

A large high-altitude object was spotted on 28 January as it passed over Alaska's Aleutian Islands, an archipelago in the northern Pacific Ocean between the US and Russia.

It was then tracked entering Canadian airspace before emerging over the western US state of Montana.

On 1 February, photos taken by shocked residents in the city of Billings first brought the public's attention to the high-altitude object after it had already shutdown their local airport.

Montana, a sparsely populated state, is home to one of only three nuclear missile silo fields in the country, and officials said the object was a suspected Chinese surveillance balloon that appeared to be monitoring sensitive sites in the region.

Officials said it was over 200ft (60m) tall and was equipped with multiple antennas, solar panels and surveillance equipment capable of intercepting telecommunications.

But officials at first declined to shoot it down due to concerns about the damage falling debris could cause, so it was allowed to drift across the continental US for days.

On 4 February, President Joe Biden authorised it to be shot down by an F-22 jet off the coast of South Carolina.

Recovery crews collected some debris and are using boats and mini-subs to reach further equipment from the balloon, which is submerged in about 47ft of water.

China denied that it was used for spying, saying it was a weather monitoring device that was blown astray.

The balloon's destruction "seriously violated international practice," Beijing said, adding that it reserved the right "to use necessary means to deal with similar situations".

Alaska and Yukon

On Friday 10 February, another object was taken down by US forces off the coast of northern Alaska.

Officials said it was "the size of a small car" and had been flying 40,000ft (12,000m) in the air as it travelled in the direction of the North Pole without any system of propulsion or control.

The next day, on Saturday 11 February, a similar "high-altitude airborne object" was shot down on US and Canadian orders over the Yukon Territory in north-western Canada.

It is still not clear what these two objects were and reports give a mixed picture.

Senator Chuck Schumer, the top Democrat in Congress, said on 12 February that intelligence officials believed they were both balloons, "but much smaller than the first one".

Asked about Mr Schumer's comments, a spokesman for the US Department of Defense said the two "did not closely resemble" the original balloon.

And one senior official told ABC News that the most recent objects to be shot down were likely weather balloons and not surveillance devices.

Unidentified flying objects - timeline

4 February: US military shoots down suspected surveillance balloon off the coast of South Carolina. It had drifted for days over the US, and officials said it came from China and had been monitoring sensitive sites

10 February: US downs another object off northern Alaska which officials said lacked any system of propulsion or control

11 February: An American fighter jet shoots down a "high-altitude airborne object" over Canada's Yukon territory, about 100 miles (160 km) from the US border. It was described as cylindrical and smaller than the first balloon

12 February: US jets shoot down a fourth high-altitude object near Lake Huron "out of an abundance of caution"

On Sunday 12 February, US jets shot down another object as it neared Lake Huron, one of the largest of the Great Lakes that straddle the US-Canada border.

It marked the third object shot down in as many days and much like the previous two - it is not clear what this was.

Defence officials described it as an octagonal structure with strings hanging off it. They said it was blown up "out of an abundance of caution" because it posed a hazard to civilian aviation due to the height it was flying out.

After the latest incident, the US Air Force commander tasked with guarding North American airspace said he could not explain what the three most recent objects were, how they stayed aloft, or where they were from.

And asked about the possibility of extraterrestrial activity, General Glen VanHerck said nothing had been ruled out.

Officials say US radar systems - which are more equipped for fast-moving missiles than hovering balloons - may detect more objects as their search continues.

"Now, of course, we're looking for them. So I think we're probably finding more stuff," Congressman Jim Himes, the top Democrat on the House Intelligence Committee, told NBC News.

Recovery and reaction

Teams are working to recover the debris from the objects.

Helicopters and transport aircraft have been deployed to the frozen ice of the Beaufort Sea, off the northern coast of Alaska.

"Arctic weather conditions, including wind chill, snow and limited daylight, are a factor," the US military said.

In the Yukon, Canadian CP-140 patrol aircrafts are searching for debris from the third object.

Meanwhile, a large portion of the original balloon has already been recovered from the Atlantic Ocean, and is being analysed by FBI agents.

As thing stand, there are growing calls for the White House and defence officials to provide more information as soon as possible.

"We need the facts about where they are originating from, what their purpose is, and why their frequency is increasing," Michigan Democrat Debbie Dingell said.

-bbc