Recent acts of deadly extremist violence have been carried out either by US citizens or by individuals whose families weren't from the nations singled out. And the list of countries in Trump's order doesn't include Saudi Arabia, where most of the Sept. 11 hijackers were from, or other places with a more direct link to terrorism in America.

The admissions ban announced Friday also does not directly address a more urgent law enforcement concern: homegrown violent extremists already in the United States who plot their attacks without any overseas connections or contacts.

"The primary terrorism-related threat facing the US today comes from individuals living here who become inspired by what they see on the internet, who carry out attacks independent of any terrorist organization," said John Cohen, a former Department of Homeland Security counterterrorism official who worked in government under Democratic and Republican administrations and who has been involved in refugee vetting policy.

The FBI has for years been concerned by the prospect of airplane bomb plots and terrorists dispatched from overseas to commit violence in America. But the ascendancy of the Islamic State, and the group's ability through slick and easily accessible propaganda to reach susceptible young Americans in all corners of the country, has been a more immediate challenge — and a more realistic danger — for counterterrorism officials than any threat posed by refugees from abroad.

"Dealing with that threat should be a top priority for this administration," Cohen said.

The executive order suspends refugee admissions for 120 days and bars all immigration for 90 days from Muslim-majority countries with terrorism concerns: Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen.

It indefinitely bars the processing of refugees from Syria, a country that's been of particular country to the FBI even though the number of Americans who have looked to travel there to fight with the Islamic State has been dwindling.

But the culprits of recent deadly terror attacks aren't linked to the countries singled out by Trump's order.

Omar Mateen, the man responsible for the Orlando nightclub shooting, the deadliest terror attack in the US since the Sept. 11 attacks, was born in New York to Afghan parents.

Syed Rizwan Farook, who took part in the December 2015 San Bernardino attack, was born in Chicago. His wife, Tashfeen Malik, had been living in Pakistan and visiting family in Saudi Arabia before she passed the background check and entered the US.

The brothers who bombed the Boston Marathon were ethnic Chechens who had been living in the US.

In general, Islamic extremists have accounted for a minuscule amount of the roughly 240,000 murders since Sept. 11, 2001, said Charles Kurzman, a sociology professor at the University of North Carolina at Chapel Hill.

Kurzman said his research identified zero fatalities since Sept. 11 caused by extremists from the seven nations in Trump's order. He said people with ancestry from those countries have accounted for only a small fraction of extremist-related arrests and disrupted plots during the same time period.

"I can only conclude that this is whipping up fear and hostility toward Americans who have family background from these countries," Kurzman said.

Still, while refugees are subject to screening — including in-person interviews, checks with law enforcement databases and collection of biometric data, when available — the process is not perfect.

FBI counterterrorism officials have long expressed concern about the lack of background information about refugees from Syria, a home base of the Islamic State, and Director James Comey has said that he could not guarantee a mistake-free vetting process.

There have been isolated incidents of refugees later accused in terror-related plots.

An Iraqi refugee who entered the US in 2009, for instance, pleaded guilty in Houston in October to attempting to provide material support to the Islamic State. Two Iraqi refugees who lived in Kentucky are now in prison after having been convicted in a plot to send sniper rifles, Stinger missiles and money to al-Qaida operatives waging an insurgency back home.

And the man accused in the November car-and-knife that injured 11 people at Ohio State University was a refugee originally from Somalia who, as an adolescent, moved with his family to the United States in 2014 after living in Pakistan.

Though not immune from lapses, the screening process has improved over the years, Cohen said. He said he was concerned that the refugee ban could deter Muslim-majority countries from cooperating with the US on policy matters and could embolden an extremist already bent on violence.

"That's something," Cohen said, "that law enforcement folks are going to be factoring into their violence prevention efforts."

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