A massive spy balloon believed to be from China was seen above Montana and is being tracked as it flies across the continental United States, with President Joe Biden for now deciding against "military options" because of the risk to civilians, U.S. officials said on Thursday.
Still, officials insisted, they continue to closely monitor the vessel as they have since it entered the country -- while voicing their concern to the Chinese.
"The United States government has detected and is tracking a high-altitude surveillance balloon that is flying over the continental United States right now," Brig. Gen. Pat Ryder said in a statement on Thursday.
"NORAD (North American Aerospace Defense Command) continues to track and monitor it closely," Ryder said.
The high-altitude reconnaissance balloon was not the first such craft to pass over the U.S. in this way, a senior defense official said in a briefing.
A separate senior official told ABC News the balloon is the size of three buses and complete with a technology bay, which the defense official said they "wouldn't characterize" as "revolutionary."
The defense official said they "are confident" the balloon was sent by China.
"Instances of this activity have been observed over the past several years, including prior to this administration," the official said, noting that "it's happened a handful of other times over the past few years .... It is appearing to hang out for a longer period of time this time around."
One outside expert predicted the balloon could be essentially scientific and set off course, though other sources said it appeared to be "purposeful."
Retired Col. Steve Ganyard, an ABC News contributor, said the balloon looked to be a standard research vessel -- which would mean it was unpowered and drifted with the jet stream.
A senior U.S. official, however, told ABC that the military balloon was specially designed, with a purposeful path. They believe the balloon can take high-resolution pictures and is flying along areas where there are missiles and bases.
Echoing that, the defense official told reporters: "I'm not trying to be a wise guy, but the goal was surveillance and clearly they're trying to fly this balloon over sensitive sites ... to collect information."
The defense official said the U.S. had used "multiple channels" to tell China how seriously they are taking this incident. "We have made clear we will do whatever is necessary to protect our people and our homeland," said the official, who added that if the risk does increase then "we will have options to deal with this balloon."
Biden was briefed about the balloon and "asked for military options," the defense official said. The president agreed with the recommendation of the chairman of the Joint Chiefs of Staff, Gen. Mark Milley, and U.S. Northern Command Gen. Glen D. VanHerck to not "take kinetic action due to the risk to safety and security of people on the ground from the possible debris field."
A senior administration official echoed that view and said in a statement, "We acted immediately to protect against the collection of sensitive information."
The balloon was seen over Montana on Wednesday and military aircraft spotted in the sky above Billings were U.S. Air Force F-22s, according to a U.S. official.
The Federal Aviation Administration confirmed that there was a ground stop in Billings on Wednesday but an agency spokesperson did not share more details.
The senior defense official said the presence of the F-22s and the FAA's ground stop were put in place in case the decision was made to shoot down the balloon.
"We didn't take the shot, but that's why you saw the reports that you saw," the official said.
Why not shoot? reporters asked. "The first question is, does it pose a threat?" the official said. "Our assessment is it does not," they said.
"We have been tracking it for some time. And we have had custody of it the entire time. It's been over U.S. airspace, entered the continental United States' airspace, a couple days ago," the official said.
"Currently, we assess that this balloon has limited additive value from an intelligence collection perspective," the official said. "But we are taking steps nevertheless to protect against foreign intelligence collection of sensitive information."
The official wouldn't say where the balloon is now. But they said it is not posing a risk to civilian aviation because it's at such a high altitude -- "significantly above where civilian air traffic is active."
Military expert's view
Ganyard predicted the balloon was an experiment gone awry.
Such balloons are not controlled after their release and while they are normally equipped with mechanisms to deflate over an open area, the mechanisms can fail, Ganyard said. So it's possible the balloon would have drifted over from China after multiple days, rather than being nefariously deployed.
China intentionally deploying a reconnaissance balloon over the U.S. would be highly provocative, with little value, Ganyard said, noting that Chinese satellites are able to collect information in a similar manner.
Regardless, the senior defense official said on Thursday that "we know exactly where this balloon is, exactly what it is passing over, and we are taking steps to be extra vigilant so that we can mitigate any foreign intelligence risk."
Members of Congress on Thursday pushed for more answers.
"The Department of Defense owes Congress and the American people a full and accurate accounting of why U.S. forces did not take proactive measures to address this Sen. Roger Wicker, R-Miss., the ranking member of the Armed Services Committee, said in a statement. Wicker argued that "information strongly suggests the Department failed to act with urgency."
Montana Republican Sen. Steve Daines requested a security briefing from the Biden administration. In a letter to Defense Secretary Lloyd Austin, Daines called it "imperative" that the government affirm the American people's safety and security.
A spokesperson for Montana's Democratic senator, Jon Tester, said he was "monitoring this situation closely and will continue to receive updates from DOD."
The top Republican and Democrat on the House's select committee on China also shared their outrage, saying in a statement that the balloon represented a "violation of American sovereignty."
"This incident demonstrates that the CCP threat is not confined to distant shores-it is here at home and we must act to counter this threat," Chair Mike Gallagher, R-Wis., and ranking member Raja Krishnamoorthi, D-Ill., said.
ABC News' Amanda Maile, Jay O'Brien, MaryAlice Parks, Allison Pecorin and Trish Turner contributed to this report.