This Sunday marks the 15 year anniversary of September 11—the worst terrorist attack on American soil. It was perpetrated by 20 al-Qaeda-affiliated men primarily from Saudi Arabia, and since then, fears of foreign terrorists have been understandably widespread.

But not one domestic terrorist attack since 9/11 has been committed by a foreign terrorist organization. Overall, terrorism in America is happening from homegrown radicals.

"There is a conventional wisdom that terrorism in the US is the province of foreigners and is seen as a problem of infiltration," says David Sterman, a senior program associate with the international security program at the New America Foundation. "And while there is certainly a reason for that perception, as the September 11 attacks were conducted by people who came in from abroad, in the more than 360 cases [of jihadist terrorism] we’ve examined since September 11, we found 80 percent are US citizens and legal residents."

US citizens and legal residents have committed 80 percent of terrorist attacks since 9/11

September 11, 2001 is still the worst international terrorist attack that has ever happened on American soil, killing more than 3,000 people and wounding countless others. (Since then, 142 Americans have died from terrorism at home.) But 9/11 is an outlier in another key way, too. Every terrorist attack in the US since 9/11 has been committed by people living here and not by outside foreign terrorist organizations, including the latest mass shooting in Orlando, Florida. Omar Mateen was an American citizen.

In the same time frame, however, there have been 28 deadly domestic terrorist attacks perpetrated by homegrown terrorists. Homegrown terrorism commonly refers to terrorist acts committed by a government's own citizens. While sometimes used to describe an Islamic extremist threat, homegrown terrorism isn't tied to any one ideological background.

"In the more than 360 cases we've examined since September 11, we found 80 percent are US citizens and legal residents"



Of the 28 deadly homegrown terrorist attacks, only 10 of those attacks were related to Islamic extremism. The other 18 attacks were led by right-wing extremists, including, most recently, the mass shooting on November 27, 2015 that killed three and wounded nine at a Planned Parenthood in Colorado Springs, Colorado.

The data from New America clearly shows that domestic terrorist attacks have, since 2001, been more commonly perpetrated by right-wing groups than by Islamic extremists. But when you look at data on arrests, the story flips: Since 2001, there have been 364 charges in the US for "jihadist terrorism," which New America defines as violent extremism motivated by al-Qaeda and its affiliated groups, and only 182 for right-wing terrorism.

Sterman says one reason for this discrepancy is it is often hard to obtain reliable data on right-wing terrorism, as the government doesn't always label it as terrorism, whereas jihadist terrorist activity is almost always labeled as some form of terrorism. When deciding whether to classify a violent attack as a terrorist attack, Sterman said New America focuses on whether the attack was linked to a political group or organization — meaning many hate crimes often wouldn't fall under this kind of categorization. The presence of large weapon caches is another criterion used in their assessment, as Sterman said it speaks to the organizational aspect of the attack.

Most acts of domestic terrorism are committed by US citizens

Regardless of the type of terrorist attack — jihadist or right-wing — the overwhelming majority of terrorist attacks are committed by US-born citizens. Of the 546 individuals arrested for terrorism, 63 percent were born here. As my colleague Jennifer Williams notes, citizens living at home can radicalize and become terrorists too, even though that issue gets less attention. And as the data from New America confirms, terrorist acts committed by US citizens—not by refugees or foreigners–has formed the narrative of domestic terrorism since 9/11.

The New America data set does show that since 2001, 12 refugees have been charged with terrorism-related crimes. That number is still dwarfed, however, by the 346 American citizens charged with the same crime.

But what gets lost in the national conversation on domestic terrorism is that terrorist acts are largely perpetrated by American citizens already living inside America's borders. Radicalization — whether it's jihadist extremism or right-wing extremism — more often than not starts at home in the US rather than entering the country from abroad.

Watch: ISIS videos are sickening. They're also really effective.