AMMAN and ISTANBUL — Abu Rahim knows that in many ways, he's exactly the kind of potential terrorist Western governments most fear.

The 29-year-old Jordanian fought last year with ISIS, receiving specialized training in explosives in Syria. Then he returned home to Zarqa, Jordan, before being accepted to an engineering school in Romania, where he lives today on a student visa.

"I feel like I could travel freely in Romania — and to Europe. I feel like once you're there, the borders are not so secure," he told BuzzFeed News in a phone interview from Bucharest, asking to go by his wartime nickname to avoid being identified by authorities as someone who fought with ISIS. "I'm not a bad guy; I'm not a terrorist. But I am aware, in the news, that they are worried about men like me."

In the wake of the attacks in Paris, intelligence agencies worldwide have been forced to look at their security-preparedness anew. The symbolic and well-coordinated attacks of the past — such as 9/11 and the 2005 subway bombings in London — still stick out in the public mind. But government officials increasingly worry about a different and perhaps more elusive kind of threat: "Fairly low-tech attacks," as one Western official tracking terrorist groups described them, that focus on "soft targets," like a supermarket or a newspaper.

The planning of these attacks can be small-scale enough to fly under the radar — and they could be carried out by someone with limited connections among established extremist groups. And such plots become all the more lethal in the hands of a veteran of one of the world's ever-expanding jihadi battlefields: Iraq, Syria, Libya, Mali, Yemen.

It's this threat exactly that the Paris attacks — and ensuing wave of arrests elsewhere in Europe, some targeting recent returnees from the war in Syria — hammered home.

Western officials present it as a dangerous hybrid of the "lone wolf" model of terrorism and the high-profile attacks involving significant resources and planning. "I think what we're looking at now is not 'lone wolf' necessarily, nor is it the command-and-control from structured, hierarchy-based organizations as we have seen in the past," said Alexander Evans, coordinator of the UN Security Council expert group on al-Qaeda and associates. "It's just flatter and peer-to-peer, inspired by radicalization networks and maybe loosely linked to them."

Terrorist groups seem to have their sights set on this mode of attack. In recent weeks, extremist leaders in Iraq, Gaza, and Yemen have renewed their calls for sympathetic Muslims to carry out attacks in Western countries.

Among those leading the calls was Nasser bin Ali al-Ansi, an official with the Yemen-based al-Qaeda in the Arabian Peninsula, which claimed credit for the deadly rampage at the Charlie Hebdo newspaper in Paris. One of the two brothers responsible for the attack reportedly traveled to Yemen in 2011, where he either fought with or received training from the group. "If he is capable to wage individual jihad in the Western countries that fight Islam ... then that is better and more harmful," Ansi said in remarks to the group's media arm, according to the SITE Monitoring group.

It's an idea that's also being discussed by members of extremist groups on the ground. An official with Jabhat al-Nusra, al-Qaeda's affiliate in Syria, said it would be "easy" for the Western citizens fighting with the group to launch attacks at home. "All the foreign jihadis fighting with Nusra, they can go back to their home countries, and it's easy for them to make bombs and find weapons," the official, who goes by the nom de guerre Abu Mariya al-Souri, said in a phone interview from the Turkey-Syria border. "They've spent years fighting. They've learned everything about how to wage jihad. And the Western governments don't know who went to jihad and who came back."

Western governments are faced with a difficult challenge in such a scenario: Potential attackers may be more capable than a typical "lone wolf," while operating with a relative independence that makes them difficult to track. Battlefields like Syria, meanwhile, continue to draw in more recruits. The U.N. has estimated that some 15,000 foreigners have traveled to Syria to fight with ISIS and other extremist groups.

"It's not as if the threat of more organized attacks has gone away," said Evans, the UN expert on terrorist groups. "But the challenge here is the very large numbers, and it's not always people who are known to governments."