At this stage, there is no hard evidence. But reports from the U.S. and Britain suggesting an Islamic State group bomb may have caused the Russian plane crash in Egypt are raising the alarm among experts, who say such an act would be a frightening change in tactics by the extremist group.
It would also underscore the failure so far of the U.S.-led coalition to deter the jihadis - despite the recent addition of Russia to the seemingly formidable forces arrayed against them.
Russian and Egyptian officials say any talk about a bomb is premature, and aviation authorities are working on all possible theories as to why the Airbus A321-200 crashed Saturday in Egypt's chaotic Sinai Peninsula, 23 minutes after takeoff.
Still, British Prime Minister David Cameron said Thursday it was "more likely than not" that an explosive device brought the jetliner down. If that proves to be true, and if the Islamic State group was responsible, it would be the Sunni extremists' largest act of transnational terrorism by far.
While the Islamic State group has claimed responsibility for deadly attacks that struck tourists in Tunisia and Shiite mosques in oil-rich Sunni Gulf countries - claims that have not been proven - it has so far refrained from spectacular al-Qaida-style attacks on airliners. It has focused instead on seizing and expanding territory it already holds in Syria and Iraq, and establishing branches in other countries like Egypt and Libya.
And while some attacks in the West may have been inspired by the group, there has been no clear evidence that any of them was planned or directed by the group itself.
"The Sinai attack would be a first, and would signal that the Islamic State has become both capable of - and interested in - joining the dreadful ranks of global terrorism," concluded an analysis by the Soufan Group, a private geopolitical risk assessment company.
Given the Islamic State militants' success in creating mayhem in the region through its brutal tactics and ferocious fanaticism, such a metamorphosis would be a major challenge for security services around the world.
IS has claimed responsibility for bringing the Russian plane down in written statements, as well as video and audio messages posted on the Internet this week. It said the attack was retaliation for Russia's air campaign against IS - and other groups - in Syria, where Moscow wants to preserve the rule of President Bashar Assad. The group warned Putin that they would also target him "at home."
But IS has not offered any details to back its claim. While releasing specifics would add credibility, the group may be withholding either because its claim is false, or because doing so would undermine plans for similar attacks in the future - or because the aura of mystery might deepen its mystique among die-hard followers.
A U.S. official briefed on the matter said that intercepted communications played a role in the tentative conclusion that the Islamic State group's Sinai affiliate planted an explosive device on the plane.
However, the official added that if it was a bomb, intelligence analysts don't believe IS leaders in Syria ordered the operation, but rather that it was planned and executed by the group's Sinai affiliate, which operates autonomously.
The Islamic State group's insistent series of responsibility claims suggests it is trying to boost its global credentials.
Until now, the main advantage it has claimed over al-Qaida is its hold over a significant chunk of territory in Iraq and Syria.
Aymenn al-Tamimi, an expert on rebel and Islamic extremist groups and a fellow at the Middle East Forum think tank, said bombing a jetliner becomes a significant "one-up" in the rivalry with al-Qaida.
"If the Islamic State is capable of conducting attacks like this - particularly against a target now widely reviled (Russia) - this could bolster their appeal in the jihadi world," he said.
It a bomb brought down the plane, it would not be the first time a Russian jetliner was targeted by Islamic militants. Two suicide attackers brought down two Russian planes over Russia in 2004, killing 89 people - attacks claimed by Chechen rebels. Chechens and other militants from the northern Caucasus still have lots of reason to strike at Russian targets.
IS regularly uses high-tech propaganda videos, including those showing the beheadings of foreign hostages, as a form of psychological warfare.
Those gruesome videos have sent shockwaves across the globe and appear to have succeeded in instilling terror outside their base territory without having to actually attack - although there have been some gruesome killings around the region, including beheading a group of Ethiopian laborers in Libya.
In recent days, IS for the first time explicitly threatened Israel, in videos featuring a militant speaking fluent Hebrew. If they make good on this threat, the potential for escalation is huge.
Such ambition underscores the extreme challenge facing the U.S.-led coalition. Part of the challenge is the group's geographic dispersal: It has branched out from its base in Syria and Iraq, adding affiliates in Egypt's Sinai, Afghanistan, Libya and Yemen.
The U.S. and its allies have been bombing IS in Iraq since August 2014, extending the campaign to Syria a month later. Russia recently joined the fray, launching an aerial campaign against the group in Syria on Sept. 30. Iranian-backed militias in both countries are fighting the group on the ground. Yet IS has not been pushed back an inch in its strongholds of Mosul in Iraq and Raqqa in Syria.
In another incremental step, Obama announced last week the U.S. would deploy up to 50 U.S. special operations troops into northern Syria to assist in the fight against IS.
The question now becomes whether a bomb planted by IS on an airliner will spur more serious and decisive action to destroy the group.
An official with the U.S.-led coalition in Iraq said a bomb on an aircraft is a somewhat isolated incident and would be hard to replicate, adding that it wouldn't necessarily demand a change in coalition tactics.
The attack will play into Putin's narrative that IS needs to be fought in Syria now, before it poses an even bigger threat to Russia.
Putin has regarded Syria as an opportunity to promote Russia's world standing at relatively low cost and risk, but will now be keeping a close eye on public opinion at home.