Refugees will turn into terrorists. That seems to be the consensus among those in Congress opposed to refugees. But history gives us no reason to believe it. No refugee has ever carried out an act of terrorism in United States. In fact, anti-immigrant activists have proven themselves more prone to terrorist tactics than refugees. The risk of terrorism is just a callous excuse to turn away vulnerable people fleeing violence.

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The United States has welcomed almost 2 million refugees since 1990, including hundreds of thousands from the Middle East. There have been dozens of terrorist attacks on American soil in that time, but none of them involved refugees.

The 1997 Empire State Building shooter, a Palestinian, was in the country on a nonimmigrant visa. The 9/11 attackers all arrived in the United States on student or tourist visas. Neither of the 2002 Beltway snipers were refugees. The 2015 Chattanooga, Tenn., shooter, a Jordanian born in Kuwait, gained American citizenship as an infant.

The majority of U.S. terrorism is homegrown. The 2006 Seattle Jewish Federation shooter was a native-born American, as was the 2009 Little Rock, Ark. recruiting office shooter. The 2009 Fort Hood, Texas shooter was born in Virginia. A Kansan was responsible for the 2013 Wichita airport bomb attempt. The three men who carried out the attack on the 2015 "Draw Muhammad" contest in Garland, Texas were Americans born and raised.

A few cases of terrorism have tenuous connections to asylum claims, but only indirectly. Two 1993 World Trade Center bombers requested asylum, but never received it. (Their accomplices entered on tourist or student visas.) The 2002 Los Angeles airport shooter applied for asylum, but was denied. The 2013 Boston Marathon bombers were the young children of an asylee, but not asylees themselves.

Refugees were implicated in terrorist plans in a couple cases, but these plots were stopped long before they became reality, which should reinforce our trust in law enforcement to deal with these minor threats. In congressional testimony this summer, Foundation for the Defense of Democracies' Daveed Gartensein-Ross had to make the security case against refugees without finding a single successful terrorist attack by a refugee.

Even if you include the failed attackers, just 0.0002 percent refugees turned out to be terrorists, and they have still been less inclined to terrorism than anti-government extremists, radical environmentalists or pro-life Christians. They are actually less dangerous than some of anti-immigration zealots who want to keep them out.

In 2007, five members of an anti-immigration militia in Alabama planned a machine gun attack on Mexican workers. After they began to spy on their targets, agents raided their homes and found automatic weapons and hundreds of grenades. A Wyoming militia leader planned a similar attack the same year and was convicted of weapons charges.

In 2010, James Lee posted a screed online railing against population growth and immigration before taking three people hostage with four explosive devices at the Discovery Channel headquarters in Maryland. He called for "solutions to stopping all immigration pollution" and "anchor baby filth."

In 2014, Larry McQuilliams, an unemployed Texan with ties to the Christian identity movement, tried to burn down the Mexican consulate in Austin, Texas. Investigators reported that "he'd been upset that the couldn’t find a job and believed immigrants were given more services than he was." The Charleston, S.C. shooter, even while targeting a black church, also sought to bring attention to the "huge problem" of immigrants and Hispanics in his manifesto.

The point is not that we should fret over anti-immigration activists becoming terrorists; we shouldn't. The vast majority of activists are committed to implementing their policies nonviolently. But if we can agree on this conclusion, we must agree that refugees, who have planned and completed even fewer acts of terrorism, are a lesser threat.

None of this implies that Syrian refugees shouldn't be screened for ties to terrorists, and they are and will be run through a rigorous two-year screening process overseas. Yet even without this process, the risk of terrorism is a very weak excuse to allow Syrian men, women and children continue to drown in the Mediterranean or be killed in Syria.

If terrorism is really their concern, anti-immigration activists have more reason to fear themselves then they do refugees. The history of homegrown xenophobic violence, including the recent attack on a Hispanic man by Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE supporters, suggests that when politicians stoke native anxieties immigrants, they may, in fact, make America less safe. Casting refugees as terrorists only contributes to that toxic and dangerous dynamic.

This piece has been corrected to note that the Boston Marathon bombers were the sons of an asylee, not a refugee as previously stated.

Bier leads the immigration policy department at the Niskanen Center, a nonprofit in Washington.