The type and origin of the three unidentified flying objects shot down in the skies over North America last week remain a mystery, and the incidents have sparked more curiosity as to what else may be hovering in our airspace.

As efforts to recover debris from the three objects continue, the White House said Tuesday that the intelligence community is considering the possibility that they are being used for commercial or benign purposes. But that explanation deepens the puzzle even further and raises questions about what exactly the objects were used for. Experts say weather balloons or other scientific balloons are unlikely to veer off course or operate unnoticed for long periods of time.

The first of three unidentified flying objects was sighted Friday and then shot down from an altitude of 40,000 feet over the north Alaskan shoreline. An F-22 fighter jet shot down the second, at the same altitude, on Saturday over the Canadian territory of Yukon. The next day, a third object was shot down from an altitude of 20,000 feet. over Lake Huron, capping off the three-day revelry.

The puzzling events came about a week after a Chinese high-altitude surveillance balloon was seen drifting over Montana. An F-22 shot it down on February 4, raising diplomatic tensions and heightening security concerns.

Some initial reports suggested that the unidentified aerial objects could have been weather balloons or other high-altitude balloons used for scientific research, but no organization, company, or individual has so far been linked to them. The National Weather Service confirmed to NBC News Tuesday that none of the objects downed in recent days belonged to the agency.

Kevin Tucker, president of the Oregon-based aerospace company Near Space Corporation, said high-altitude balloons used for science are often well tracked and follow strict Federal Aviation Administration protocols.

Tucker’s company has been conducting high-altitude balloon missions for NASA, the European Space Agency and commercial partners for more than two decades. He said there is «nearly zero chance» that such a balloon would get lost or go astray enough to cause a national security incident.

“Do these just show up on the radar? No, they don’t,” she said. “They are fully tracked all the time, and very precisely. You know where they are and who is doing it. The element of surprise of one that just showed up, that just doesn’t happen.»

Even if a company or research organization loses control of a balloon, there are ways to deal with such anomalies, Tucker added. In most cases, that means redundant onboard timers that can trigger the balloon to close automatically.

“A lot of steps have been taken to make sure that these things don’t just take off and go away forever,” he said.

Near Space Corporation balloons are used for a variety of scientific purposes, including launching space capsules from high altitudes to test parachute landing systems. The firm is also investigating how the balloons could be used on missions to Mars and Venus.

But these balloons typically operate at altitudes above 80,000 feet, higher than the three objects that were shot down.

Overall, Tucker said, balloons offer a critical way to take measurements of the atmosphere. They are also essential for scientists who study weather patterns.

More than 100 weather stations in the United States release balloons twice a day to help make weather forecasts, said Holger Vömel, a senior scientist at the National Center for Atmospheric Research in Colorado. But such balloons are not designed to stay in the air for long.

“These balloons go up, they last for around two hours, they pop, the instruments fall back to the ground and that’s it,” Vömel said.

Susan Buchanan, a spokeswoman for the National Weather Service, said the agency flew a weather balloon from Nome, Alaska, on Friday, but it burst and fell 30 miles northeast of where it was launched. Another balloon flew from Fairbanks on Friday, but it burst and fell 35 miles to the northeast.

“Weather balloons typically travel only 30-35 miles downwind from their launch site and quickly ascend to about 100,000 feet in the air, where they burst. They don’t float,» Buchanan said in an email Friday.

Weather balloons are relatively small, measuring around 6 feet wide at ground level and expanding up to 20 feet at high altitude. They also often fly much higher than commercial and military aircraft.

While in the air, the balloons collect readings of temperature, wind, air pressure and humidity, Vömel said. Some have specialized purposes, such as studying ozone or mapping winds to gauge wildfire risks.

Vömel said it’s possible that a private hobbyist or a group with the right resources could launch balloons at high altitudes.

But he added that it is unlikely that the three unidentified objects were rogue weather balloons.

“That should be a crazy concept,” he said. «Anything larger than the size of a very small balloon has to be coordinated with the FAA, which means they’ll know at any time where the thing is, where it came from, and who it belongs to.»