WASHINGTON - Western security officials are increasingly worried that the Islamic State terror group may be a step ahead of their renewed efforts to stop terrorist infiltration of their countries.

Fears once centered on IS using migrant and refugee flows to sneak in highly trained operatives bent on carrying out attacks. Now they have expanded to include an equally dangerous possibility.

A growing number of officials now warn that the terror group may be looking to essentially weaponize refugees and other vulnerable immigrant populations after they have successfully crossed Western borders and passed through what look to be ever-tougher vetting processes.

“We have to be ready,” said Fabrice Leggeri, executive director of Frontex, the European border and coast guard agency, speaking prior to the deadly attack in Berlin.

“Some people might get radicalized or manipulated or used or utilized by terrorist groups after they enter the EU,” he said. “This is something where I don't have clear indications.”

A Europol report published in November, "Changes in Modus Operandi of Islamic State (IS) revisited" — is even more explicit.

“A real and imminent danger is the possibility of elements of the [Sunni Muslim] Syrian refugee diaspora becoming vulnerable to radicalization once in Europe and being specifically targeted by Islamic extremist recruiters,” the report stated. “It is believed that a number of jihadists are traveling through Europe for this purpose.”

Christmas market attack

Just how many terrorist operatives have been sent to Europe to recruit among the growing number of migrants and refugees is unclear. Europol cited German reports that, as of April 2016, there were approximately 300 cases in which jihadists tried recruiting refugees trying to enter Europe.

But there is also a sense that IS, also known as ISIS, is likely not as focused on the numbers as it is on exploiting what it sees as a potent opportunity.

“ISIS just wants to give itself options,” said Robin Simcox, a terrorism and national security analyst with the Heritage Foundation.

“It chimes perfectly with what ISIS would want to do,” he added. “It enables them to extend their foreign operations.”

Adding to the level of concern is the case this past week of Tunisian Anis Amri, who carried out a deadly attack on a Berlin Christmas market.

By most accounts, there were few signs Amri had radical leanings when as a 19-year-old, he arrived in Europe, on the Italian island of Lampedusa.

Even when he left Italy for Germany years later, to seek political asylum, authorities say his behavior was more akin to that of a criminal than of a terrorist.

Yet on Monday, the now 24-year-old Amri used a truck to plow through the crowded market, killing 12 people and injuring 56 others. Before the attack he made a video in which he pledged his allegiance to the Islamic State terror group and its leader, Abu Bakr al-Baghdadi.

A car sits inside a police line as authorities res A car sits inside a police line as authorities respond to an attack at Ohio State University, Nov. 28, 2016, in Columbus, Ohio. A man plowed his car into a group of pedestrians and began stabbing people before he was shot to death by a police officer. A car sits inside a police line as authorities respond to an attack at Ohio State University, Nov. 28, 2016, in Columbus, Ohio. A man plowed his car into a group of pedestrians and began stabbing people before he was shot to death by a police officer.

Ohio State University attack

U.S. officials are also concerned, pointing to an attack on American soil barely a month earlier — the November 28 car attack at Ohio State University carried out by Abdul Razak Ali Artan, a Somali-born refugee with legal, permanent resident status.

“I do think he did radicalize in the United States,” the chairman of the House Homeland Security Committee, Representative Mike McCaul, said at the time, voicing concern it was a vulnerability that could be exploited again.

“They can come in and be what they call 'clean' and radicalize after they're in the United States,” he said. “That's where the [U.S.] counter-radicalization program needs to be more robust.”

Yet improving security measures to prevent refugees from being targeted for radicalization is likely to be challenging, especially since terrorist recruiters often work without the need for face-to-face interaction.

“As long as the Islamic State, as long as [al-Qaida] have an external operations capability, have access to the internet, we have to be concerned,” said U.S. Homeland Security Secretary Jeh Johnson during a forum in Washington late last month. “I think there is little doubt we need to build these bridges to communities in which [IS] is trying to recruit.”

Not just refugees

Despite concerns and political rhetoric about the vulnerability of refugees in Europe and the U.S. to radicalization, there is also skepticism about the degree to which IS or other terror groups are specifically targeting those communities.

“When it comes to refugees being radicalized after they come to a host country, this is quite low in number, actually," according to Mubin Shaikh, a terrorism expert who has previously worked with the Canadian Security Intelligence Service.

According to the George Washington University's Program on Extremism, 112 people have been charged with IS-related crimes in the U.S. since March 2014. The vast majority of them were U.S. citizens or permanent residents.

“Much of ISIS's argument is, of course, that Muslims — immigrants, converts, everyone — will never be included and accepted in the West because of the very fact that they are Muslim," said Program on Extremism fellow Amarnath Amarasingam.

“This message, it could perhaps be argued, but gently, may indeed resonate more in some countries and with some communities,” he added. “But is ISIS specifically targeting immigrants? Not really.”