President Donald Trump's now-suspended executive order on immigration is based on a widely disputed premise: Loose immigration rules are allowing thousands of terrorists to slip into the United States undetected.
Shortly after a Muslim American gunman killed 49 people at an Orlando nightclub last June, Trump told supporters, "You have thousands of shooters like this, with the same mentality, out there in this country, and we're bringing thousands and thousands of them back in to this country every year."
The president echoed that claim after a federal judge halted his order Friday. "Because the ban was lifted by a judge, many bad people and dangerous people may be pouring into our country,'' he tweeted.
But experts say there is little evidence to back up that contention.
Indeed, most recent terrorist attacks in the United States have been carried out by homegrown Muslim extremists with few or no links to foreign countries, with recent immigrants and refugees accounting for a minority of mostly non-lethal plots. These same experts attribute that to stringent security procedures put in place after the attacks of 9/11 nearly 15 years ago.
"The U.S. government has been extremely effective at preventing the infiltration of terrorists into the United States," said Charles Kurzman, a sociology professor at the University of North Carolina who tracks Muslim American involvement in terrorism. "This is one of the great success stories of the post-9/11 era."
Plots on record
Using open source records, Kurzman has identified 414 Muslim Americans who have been involved in extremist plots in the U.S. since 9/11. Among them were 217 natural-born citizens, 60 naturalized citizens, 39 legal permanent residents, 38 refugees and 15 undocumented immigrants.
In total, attacks carried out by Muslim American extremists have killed 123 Americans in the U.S. since 9/11, according to Kurzman.
The majority of cases documented by Kurzman were non-lethal, ranging from attending terrorist training abroad, to conspiring to join al-Qaida and Islamic State.
According to Kurzman, more than 100 American Muslims have tried to join Islamic State, mostly during the terror group's "burst of mini popularity" in late 2014 and early 2015. A couple of dozen made it to Iraq and Syria, he said. No foreign fighter has returned to carry out an attack.
In addition, no one with a family background from any of the seven countries affected by the immigration order has been involved in a deadly terrorist attack in the U.S., though nearly 100 have been "associated" with violent extremism.
"The level of involvement by Muslim Americans in violent extremism is still quite low as compared with the overall level of murders and violence in the United States," he said, noting there have been some 240,000 homicides in the United States in the same period.
Support for ban
Nevertheless, supporters of Trump's order say it makes sense to restrict entry to the United States while the new administration reviews procedures to make sure they are adequate.
"This is a temporary halt in order to allow the U.S. to implement the necessary security measures," said Ira Mehlman of the Federation for American Immigration Reform, which advocates reduced immigration levels.
Current screening procedures are far from foolproof, he said, echoing concerns voiced by former director of National Intelligence, James Clapper, and other security officials in recent years.
"That's what the pause is intended to find out, so that we do have systems in place to make sure that we're not admitting people who might pose a danger," Mehlman said.
Alex Nowrasteh, an immigration policy analyst at the Cato Institute in Washington, has examined data going back to the 1970s. Nowrasteh estimated that 3,024 Americans were killed by foreign-born terrorists on U.S. soil between 1975 and 2015. All but 41 of those deaths occurred in the attacks of Sept. 11, 2001.
The risk of dying in a terrorist attack remains infinitesimal: The chance of an American being killed in an attack carried out by a foreign-born terrorist is 1 in 3.6 million per year, according to Nowrasteh. The likelihood of being killed by a refugee? 1 in 3.64 billion per year.
"In terms of the total threat that the U.S. faces on the homeland, it's a lot smaller than people realize," he said.
Numbers called misleading
In highlighting the terrorist threat, Trump has cited the more than 1,000 cases of Muslim American violent extremism under investigation by the FBI in all 50 states. And Senator Jeff Sessions, Trump's attorney general nominee, last year released data purporting that at least 380 of the 580 individuals convicted in terror cases since 9/11 were foreign-born.
But Kurzman and Nowrasteh said these figures paint a misleading picture. Kurzman pointed out that only a few dozen indictments per year have resulted from the FBI cases. Many cases are closed without charges, while others end up in charges unrelated to terrorism. In one notable case, Hussein Abuali and Rabi Ahmed of New Jersey were arrested on charges of conspiring to buy rocket-propelled grenades; they were indicted for stealing two truckloads of cereal.
Nowrasteh said that 241 out of 580 terror cases cited in the Sessions report were for offenses "related to terrorism," even though "there is no such thing as 'terrorism-related' in the U.S. law."