The shooting in El Paso, Texas on Saturday that left 22 dead is being investigated as a case of "domestic terrorism," the US Attorney for the Western District of Texas said Sunday.

Another shooting in Dayton, Ohio left nine dead on Sunday, plus the shooter. The motive for the attack is still under investigation.

The two attacks have brought attention back to the rise of mass shootings in the US, and the fact that more Americans have been killed by domestic terrorists in recent years than international terrorists.

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Two mass shootings over the weekend have reignited a national conversation on the rise of domestic terrorism and white nationalism.

On Saturday, authorities said a gunman ambushed a Walmart in El Paso, Texas, and killed 22 people after posting a screed online about a "Hispanic invasion of Texas." The deadly incident is now being investigated as a case of "domestic terrorism," the US Attorney for the Western District of Texas said Sunday.

A shooting on Sunday in Dayton, Ohio, left nine dead, plus the shooter. The motive for that attack is still under investigation.

In May, an FBI official spoke in front of the House Homeland Security Committee and revealed that, since 9/11, more Americans have been killed by domestic terrorists than international terrorists, "Good Morning America" pointed out on Monday.

Read more: There have been 255 mass shootings in the US so far in 2019 — here's the full list

Michael McGarrity is pictured above in June. Jacquelyn Martin/AP

The FBI defines international terrorism as involving subjects who are members of, or receive support from, a foreign terrorist organization. Domestic terrorists may be inspired by foreign terror groups, but are radicalized primarily in the US and do not receive direction from said groups.

"The FBI assesses domestic terrorists collectively pose a persistent and evolving threat of violence and economic harm to the United States. In fact, there have been more arrests and deaths in the United States caused by domestic terrorists than international terrorists in recent years," Michael McGarrity, the assistant director of the FBI's Counterterrorism Division, told the committee.

McGarrity didn't specify the numbers of arrests and deaths for domestic vs. international terrorism in the US.

But, as McGarrity explained at the time, the FBI is somewhat handicapped in how they can root out domestic terrorists since domestic terrorism isn't a crime in and of itself, unlike foreign terrorism.

For example, the FBI can immediately open an investigation if a person expresses support for al Qaeda online, but the same can not be done for someone who aligns themselves with a white nationalist group.

"In line with our mission to protect the American people and uphold the Constitution of the United States, no FBI investigation can be opened solely on the basis of First Amendment-protected activity. Thus, the FBI does not investigate mere association with groups or movements.

"In order to predicate a domestic terrorism investigation of an individual, the FBI must have information that the individual is perpetuating violent, criminal actions in furtherance of an ideology," McGarrity explained.

At that same testimony in May, it was revealed that the FBI had 850 domestic terrorism cases open, CNN reported.