They also represent a possible turning point in what is now a generational battle against terrorism: ISIS may have made the decision to escalate from military operations aimed at creating a caliphate in Iraq and Syria and inspiring followers to stage isolated terror attacks on the West to attacking soft, civilian targets in mass casualty strikes.
President Barack Obama said Thursday it's possible a terrorist bomb brought the plane down.
"I think there's a possibility that there was a bomb on board," Obama said in an interview with Dave Ross of CBS News affiliate KIRO in Seattle.
He said the current intelligence isn't definitive enough to say exactly what felled the aircraft and noted that security procedures in place in the region were different than in the United States.
"We're going to spend a lot of time just making sure our own investigators and own intelligence community find out what's going on before we make any definitive pronouncements," he said. "But it's certainly possible that there was a bomb on board."
The disaster, which killed more than 200 mainly Russian civilians on a flight from the Egyptian resort of Sharm el-Sheikh to St. Petersburg, also presents the United States with a new flurry of complicated security, diplomatic and political questions.
The U.S. government has largely kept American civilians safe from terrorism during the 14 years since 9/11, and President Barack Obama has claimed that record and the crushing of core al Qaeda under his watch of one of his top achievements.
But the possibility that ISIS is behind the plane crash raises the specter of a new potential for devastating attacks on Americans. If another Islamist group has acquired the motivation and the capacity to attack civilian airliners, a future target could be U.S. jets.
"It is a long war and you we have just seen maybe a very significant turn and escalation in that war," said Aaron David Miller, a former U.S. Middle East peace negotiator now with the Wilson Center in Washington.
"We haven't yet found a way -- and I think it's going to be extremely difficult to hermetically seal the nation, abroad or here at home, against these kinds of attacks," he said on CNN's "New Day."
How to respond to the possibility of an expanded ISIS threat -- if confirmed by investigators -- will likely vex the administration and increase pressure on U.S. intelligence agencies to forestall a similar strike against a U.S. target.
An expanded and potentially more lethal threat from ISIS to Americans could also scramble the assumptions that are currently underpinning Obama's policy towards the region.
Guided by his desire to extricate the United States from costly, intractable foreign wars, he has been loath to jump into new ones.
Initially, he ridiculed the idea that ISIS would evolve into the same kind of global threat as al Qaeda, once referring to the group as a junior varsity organization. The President has also argued that the events of the last decade prove that Washington has a limited capacity to dictate outcomes in a Middle East torn by conflict, where traditional borders and ideas of nationhood are being ripped apart in a tide of sectarianism.
So his decision to deploy troops back to Iraq after ending the war, firstly to train and assist Iraqi forces and then to use air power to target ISIS, were taken reluctantly. And the President's decision last week to send 50 Special Operations forces to help U.S.-backed rebels in Syria -- putting boots on the ground, as he eventually did in Iraq as well -- was a step that he had previously told Americans he would never take.
Now, the White House might be forced into yet another exhaustive review of its goals and strategies in the region.
Any confirmation that the plane was downed by ISIS in retaliation for Russia's intervention in Syria to prop up the group's enemy President Bashar al-Assad will bring its own complications for Moscow -- and for Washington.
A confirmed attack by ISIS would place tremendous political pressure on President Vladimir Putin, just weeks into his military intervention in Syria, if ISIS or another group has already been able to make Russian civilians pay a heavy price. Putin is trying to protect a Russian client in the most sweeping action in the Middle East by Moscow since the end of the Cold War, but he might face blowback if the upstart terror group has landed such a crushing blow.
In the past, Putin has reacted to domestic political pressure by stoking nationalism and by acting decisively abroad, often in conflict with U.S. interests. So it's very difficult for the United States to predict exactly how he will react.
One option for Putin might be to double down on his policy toward Syria and in support of Assad, raising the possibility of new conflicts with U.S. operations in the country.
At the same time, it could solidify Russia's view that ISIS is an implacable enemy, as it is for the United States. While Moscow has said that it has been focusing largely on ISIS targets in Syria, the Pentagon has said most of the attacks have been on rebels fighting Assad. The U.S. and Russia might now find themselves more in common cause.
At the very least, it is clear that Putin has a headache on his hands -- which could partly explain why Moscow has downplayed the possibility that a bomb caused the jet's demise despite multiple Western sources pointing to terrorism as the likely cause. Given the obfuscation and refusal to accept the Western version of the shooting down of a Malaysia Airlines plane over Ukraine by suspected pro-Russian rebels last year, there is no guarantee that there will even be an definitive common view of what happened to the Russian jet.
In that sense, the White House already has a foreign policy challenge in establishing exactly what happened over the Sinai.
"Their track record when it comes to these kinds of things -- when it comes to the Russians at least -- is not very good," White House spokesman Josh Earnest said Thursday.
Still, although it's unlikely to be expressed in public, some U.S. officials might also privately feel vindication for Obama's recent warning that Putin was stepping into a "quagmire" in Syria, one that the President's policy of limited involvement has been designed to avoid.
"This is a major game changer, because for over a year, many people in the United States, politicians on the right and the left, have been calling for pouring more military forces or doing more militarily against ISIS in Iraq and Syria," said Robert Pape of the Chicago Project on Security and Terrorism at the University of Chicago.
"Well, the Russians finally did that and what we have seen is, the more military intervention occurred, the more ISIS makes it a priority to kill civilians from those states," Pape said on CNN.
At home, however, Obama will likely face calls for further engagement on the issue from his political opponents.
Republican presidential candidates in particular have been lacerating Obama over his decision not to intervene more decisively in Syria, arguing that allowing ISIS to grow will eventually create a group with the willingness and capacity to extend its reach to attack U.S. and western targets.
Obama's decision last week to dispatch Special Forces to Syria has already drawn derision from Republicans as being insufficient to counter the magnitude of the threat.
And the GOP is likely to only escalate its accusations that Obama is weak and presiding over a disastrous foreign policy that has emboldened terrorists and endangered the United States.
"He is going to be hammered -- and he already is on the campaign trail by Republicans who argue, 'See, Mr. President, you identified this as a JV organization, you are risk averse. What if this had been an American aircraft?" Miller said.
The attacks on Obama started within hours of first reports from London that the Russian airliner might have been downed by a bomb, which CNN then reported was likely orchestrated by ISIS.
"You're not going to win this war from the air. You need a ground component. There are not enough left, in my view, Syrian Arabs and Kurds to do the job," said Republican presidential candidate Lindsey Graham, a South Carolina senator.
"You need a regional force. The strategy President Obama has charted will not work," Graham continued. "It's just a matter of time before they come here. I have been saying that for two years now. We got to get these guys in a box and nail it shut."
Another GOP candidate, former Florida Gov. Jeb Bush, said that Obama was guilty of treating the campaign against ISIS like a "law enforcement" program.
"Every day they exist in the form of a caliphate is another day they gain energy, so I think the President needs to acknowledge this is a threat to our national security interests," Bush said in New Hampshire on Wednesday evening. "I believe we need to take out ISIS and create a more stable Iraq and a more stable Syria. American leadership is required for that to happen."