Here is what we know so far:

- What were the four objects? -

The drama began in late January, when a giant Chinese balloon -- dubbed a spy craft by American officials -- drifted for days through US skies before being shot down February 4 by an F-22 jet off the South Carolina coast. China insisted the balloon was conducting weather research and had gone astray.

The Pentagon said it had a gondola the size of three buses and was equipped with multiple antennas and had solar panels large enough to power several intelligence-gathering sensors.

It also appeared to be able to steer itself, using winds and possibly a propulsion mechanism, officials said.

Then Friday, US fighter jets downed another object off northern Alaska. It was much smaller than the Chinese balloon, and lacked any system of propulsion or control, officials said.

On Saturday, a US F-22 jet, acting on US and Canadian orders, downed a "high-altitude airborne object" over Canada's far northwest Yukon territory, saying it posed a threat to civilian flight. Canada described it as cylindrical like and about the size of a Volkswagen Beetle.

On Sunday Biden ordered US warplanes to down yet another unidentified object over Lake Huron. The object was described as an octagonal structure with strings hanging off it.

It too posed a hazard to civil aviation as it flew at about 20,000 feet (6,000 meters), officials said.

The Pentagon said none of the four objects appeared armed or posed any threat of attack.

Officials would not comment on the origin or function of the three objects that came after the Chinese balloon. Only the balloon has been attributed to Beijing so far.

- What has been recovered? -

Military teams working from planes, boats and minisubs are scouring the shallow waters off South Carolina for the first object, and military images showed the recovery of a large piece of balloon. The Federal Bureau of Investigation is taking custody of the debris for analysis.

Operations to recover the second object continue on sea ice near Deadhorse, Alaska. "Arctic weather conditions, including wind chill, snow and limited daylight, are a factor," the military said.

Recovery teams are searching for debris from the third object in the Yukon, Canadian Defense Minister Anita Anand said Saturday.

The Pentagon said Sunday that US and Canadian teams were preparing an operation to try and recover the fourth object.

- What was the objects' purpose? -

US officials say the Chinese balloon, which flew over sensitive US nuclear missile sites, had surveillance equipment that could intercept telecommunications.

They said that such balloons skirted US territory at least four times in the past six years, but none had flown deep into US territory.

The balloons were part of a "fleet" operated by China -- likely the Chinese military -- that has conducted surveillance on some 40 countries over five continents, US officials said.

Speculation was growing about the other objects. US Assistant Secretary of Defense Melissa Dalton said it was known that public and private research bodies launch their own research craft into the atmosphere at those altitudes.

But after decades of investigating some strange sightings by US military pilots that were categorized as "UFOs", officials were not ready to completely dismiss the idea that aliens or extraterrestrials were the source.

"I haven't ruled out anything at this point," said US Northern Command Commander General Glen VanHerck Sunday when asked about that possibility.

- Why so many objects now? -

Dalton said Sunday that after the Chinese balloon was detected, US air defense made adjustments to radar systems to be able to detect smaller and slower-moving objects in the atmosphere.

Analysts said normally US and Canadian intelligence constantly receive huge amounts of raw data, and generally screened some out to focus on the threat of incoming missiles, not slow-moving objects like balloons.

After the radar adjustments, "What we are seeing is very, very small objects that produce a very, very low radar cross section," said VanHerck.

- What's the impact on US-China ties? -

The United States scrapped a visit to China by Secretary of State Antony Blinken, intended to stabilize severely strained relations, and has sanctioned six Chinese entities believed to support military spy balloon programs.

Beijing denounced the first balloon's downing, saying it "seriously violated international practice." It reserved the right "to use necessary means to deal with similar situations."

Dalton said Sunday that after Beijing rejected US overtures for several days, that US officials have had "contacts" with China over the balloon. She did not characterize the contacts.