In recent months, both Congress and the FBI have grown increasingly vocal about concerns focused on the swelling prominence and influence of white supremacist ideology, especially on the internet. But at times, lawmakers and law enforcement officials have been "talking past each other," as the head of the FBI's Counterterrorism Division, Michael McGarrity, recently put it.
Here are seven key questions often asked about domestic terrorism:.
How is 'domestic terrorism' defined?
"Domestic" terrorists have nothing to do with international terrorism. They are moved to violence by what McGarrity called “domestic influences, such as racial bias and anti-government sentiment.”
"Homegrown" terrorists, on the other hand, fall under the FBI's international terrorism program and are radicalized by overseas groups like ISIS or Al Qaeda.
The FBI is currently investigating about 850 possible domestic terrorists and tracking another 1,000 potential “homegrown violent extremists,” according to McGarrity and other senior FBI officials.
Is the domestic terrorism threat growing?
"Domestic terrorism notably is on the rise," McGarrity told the House Homeland Security Committee in May. "[And] the threat of domestic terrorism exists in every region of the United States and affects all walks of life."
Specifically, the number of actual attacks carried out by domestic terrorists has risen, and that is why authorities are so concerned.
But the number of domestic terrorism investigations has in fact dropped in recent years -- from about 1,000 two years ago to 850 now. And the number of domestic terrorism arrests has also dropped. In FY 2017, about 150 of the FBI's domestic terrorism subjects were arrested, according to the FBI. In FY 2018, the number dropped to 115, and so far in FY 2019 the number stands at 90.
As McGarrity testified: "We're actually down in cases. But ... [the] velocity is much quicker than it's ever been before."
"There have been more arrests and deaths in the United States caused by domestic terrorists than international terrorists in recent years," he said. "Racially-motivated violent extremists are responsible for the majority of lethal attacks and fatalities perpetrated by domestic terrorists since 2000."
McGarrity noted that of the 850 domestic terrorism cases currently open, about half are what he called "anti-government, anti-authority." Another 40 percent are "racially-motivated violent extremist cases," and "a significant majority" of them "are racially-motivated extremists who support the superiority of the white race," he said.
Like "homegrown" terrorists, however, "domestic" terrorists are hard to stop because they often operate alone.
"When you can go on the internet and find content that justifies what you want to do, your specific ideology whatever that ideology is, that ... makes it harder for us to detect you from a law enforcement perspective," McGarrity said.
Are hate crimes on the rise?
It's not clear. The FBI has seen a surge in reports of hate crimes from local law enforcement agencies, but with as many as 1,000 new agencies now sending data to the FBI, the FBI can't say whether there is a real uptick in hate crimes or just an uptick in data they're receiving.
"[FBI officials] are cognizant of the increase and determining whether it's an increase in data collection and the reporting, or is there actually an increase in hate crimes? I can't answer that," McGarrity noted. "I just know it's an issue they're looking at."
How much of a priority is domestic terrorism for the FBI?
FBI officials repeatedly say domestic terrorism is one of their highest priorities. Nevertheless, the vast majority of the FBI's counterterrorism efforts are devoted to international terrorists, including "homegrown" radicals inside the U.S. inspired by ISIS or Al Qaeda.
Specifically, 80 percent of the FBI's counterterrorism cases are international terrorism cases, stretching around the world, according to McGarrity. By contrast, 20 percent of the FBI's counterterrorism case are domestic terrorism cases.
Accordingly, 80 percent of the FBI's counterterrorism agents in the field are assigned to work on international terrorism cases, while 20 percent work on domestic terrorism.
Meanwhile, nearly half of terrorism suspects were flagged to the FBI by local police, state police, or members of the public, McGarrity said.
Can the FBI open an investigation based on hateful rhetoric?
No -- not if that's all there is.
As McGarrity said: "We are prohibited from reviewing, looking at First Amendment activity. So if it's speech, if it's ideology -- and it might be alarming -- we are prohibited from that."
Testifying alongside McGarrity, a senior Justice Department official, Brad Wiegmann, put it this way: "We're going to need more than just a statement, depending on what the statement says." Even just a so-called "manifesto" of grievances isn't enough, according to Wiegmann.
But if "it's a statement that indicates threats of violence, we can investigate that," and authorities can investigate "if we have additional information about the individual -- [for example] he has his manifesto but we know the person's out buying a gun, or we have a source inside that says this person we think is is turning violent," Wiegmann added.
Would the FBI and Justice Department like more powers to investigate domestic terrorism, including a 'domestic terrorism statute'?
"From my perspective, whether I'm working gangs, MS-13 or terrorism, any tool in the toolbox helps me when I'm looking at that threat everyday as to what my options are and how I can disrupt that threat before an attack," Wiegmann testified. "We're always looking to approve our authorities. And so I think we're certainly open to having a discussion with the Congress if there's interest in the Congress pursuing a 'domestic terrorism statute,' we're certainly open to having that discussion."
Wiegmann suggested that a domestic terrorism statute could mirror current hate crime statutes, which allow for harsher sentences when crimes are committed out of bias and discrimination.
"That would be something that we could do that would be broader on domestic terrorism," Wiegmann said. "Kind of like hate crimes but focused on domestic terrorism."
Wiegmann, however, noted that federal authorities currently have "a whole array of charges" they can bring against domestic terrorists to take them off the streets and deliver justice.
"We can use gun charges, we use explosive charges, we use threat and hoax charges, we can use hate crimes," he said.
But do the FBI and Justice Department want to start designating domestic groups as terrorists?
No, at least not officially. The First Amendment presents a series of challenges and concerns.
The U.S. government designates overseas groups like ISIS as “foreign terrorist organizations,” and it is a federal crime to offer them any “material support.” But Constitutional guarantees of free speech, free assembly and free association make it hard to treat groups based in America the same way.
"We probably would not want ... something that is similar to what we have on the international side, which is designating foreign terrorist organizations," Wiegmann testified. “Designating domestic groups as 'domestic terrorism organizations' and picking out particular groups that you say you disagree with their views ... is going to be highly problematic, in a way that’s not when you’re designating Al Qaeda or ISIS or an international organization."