Returning fighters from the vanquished Islamic State group pose a grave danger to Europe and the United States, but the primary extremist threat comes from people living and radicalized inside their country, US terror experts say.
Even if they have no battleground experience, those who decide to undertake solo attacks, like the two recently in New York, in the name of the Islamic State group or al-Qaeda are almost impossible to detect in advance.
"In France, the US, or elsewhere, there certainly won't be any more large attacks planned from abroad like those of November 13, 2015 in Paris," said Marc Sageman, a former CIA agent and terror expert, referring to the multipronged IS operation that left 130 dead.
"Ever since then, attackers here or in Europe have not been guided by IS but acted on their own, imagining themselves to be soldiers of an imagined Islamic community which they want to defend or avenge."
Neither Akayed Ullah, the Bangladeshi immigrant who tried to blow up a New York subway station last week, mainly wounding himself, nor Sayfullo Saipov, an Uzbek who mowed down people on a New York bike path on October 31, killing 8, had any evident contact with Islamic State jihadists aside from watching their propaganda videos.
Experts say that kind of self-radicalized attacker, completely unknown to authorities, is the main threat countries face today.
Safe haven in a bedroom
While returning IS fighters are definitely a threat, "it's not a primary concern," said Albert Ford of the New America think tank.
"The attacks in this country were made by people who were in the country for years. The real danger is with these not very sophisticated but deadly attacks that we saw lately in New York."
According to New America data, 85 percent of the 415 people accused of Islamist-related crimes in the United States since the September 11, 2001 attacks have been American citizens. Of them, 207 were born in the United States.
They also were not known to law enforcement: only one fourth had a police record.
"None of the deadly jihadist attacks in the United States since 2014 had a known operational connection to ISIS or its networks," a New America report says, using another acronym for the Islamic State group.
On both sides of the Atlantic, homegrown attacks "are obviously the most dangerous," echoed Thomas Sanderson of the Centre for Strategic and International Studies' Transnational Threats Project.
"Of the 19 last major attacks in Europe, 17 did not have a direct foreign factor element to it."
Homegrown attackers stay under the radar, giving little away that would alert police, Sanderson noted.
"Their safe haven is their bedrooms. They can prepare at home, they don't need the footprint of a camp anywhere," he said.