WASHINGTON — The news that the alleged El Paso shooter appears to have acted out of a belief in a violent white supremacist ideology has renewed calls for a federal law criminalizing "domestic terrorism."

But what exactly would that entail?

Although "domestic terrorism" is defined in the Patriot Act of 2001, there is no specific federal crime covering acts of terrorism inside the U.S. that are not connected to al Qaeda, ISIS, other officially designated international terror groups or their sympathizers.

For some time, the FBI Agents Association and some former federal officials have been advocating for one, even as the Justice Department has spent years mulling over the matter internally. For just as long, civil liberties groups have expressed opposition, fearing federal overreach and infringement on free speech.

"This may sound good, but it isn't," tweeted Hina Shamsi, who directs the ACLU's National Security Project. "FBI officials want more power than they need to investigate & prosecute white supremacist violence, ignoring or downplaying resulting harms to communities of color, Bill of Rights."

It's one thing, critics say, to attach the terrorist label to those who pledge allegiance to violent foreign groups. It's quite another to use that label in a domestic context in which even the most contemptible hate speech is protected by the U.S. Constitution.

This may sound good, but it isn't. FBI officials want more power than they need to investigate & prosecute white supremacist violence, ignoring or downplaying resulting harms to communities of color, Bill of Rights. The last thing we need is to give this govt more coercive power. https://t.co/mGqZyviE4K — Hina Shamsi (@HinaShamsi) August 6, 2019

But proponents argue that a statute can be carefully crafted in a way that protects civil rights, while giving federal authorities tools they currently lack to catalog and prevent a type of ideologically motivated violence that appears to be growing more lethal.

Prosecuting hate-motivated attackers as terrorists, they say, would send the message that the threat of extremism is just as significant when it is based on domestic political, economic, religious or social ideologies as it is when based on violent jihadism.

"From a societal perspective, should we collectively have some means whereby the federal government acknowledges the qualitative nature of that violence, and elevates it above…murder that would be routinely handled by the state courts?" Thomas Brzozowski, the Justice Department's counsel for domestic terrorism, said last year, framing the debate but carefully avoiding taking a position on the matter.

It's not that prosecutors currently lack tools to send racist killers to prison. Generally, people who kill in the name of white supremacy are prosecuted on state murder charges, and occasionally they are brought up on federal hate crimes charges.

A domestic terrorism statute, proponents argue, could be most helpful on the prevention side. It would give the FBI more legal authority to step up its intelligence gathering and investigative efforts against an amorphous group of people who are becoming radicalized mainly on the internet.

"The devil's in the details on this one," said Seamus Hughes, deputy director of the Program on Extremism at George Washington University. "These are sticky issues."

Backers of a domestic terrorism statute propose language that would make it a federal criminal offense to kill, kidnap, maim, or destroy property in a way that causes significant risk of serious bodily injury, when done to intimidate or coerce a civilian population; to influence the policy of government by intimidation or coercion; or to affect the conduct of a government. The statute also would apply to attempts and conspiracies.

That definition would appear to apply in a number of recent cases of lethal violence by people who professed to be motivated by racial and religious hatred.

The victims include six worshippers killed at a Sikh temple in Wisconsin in 2012; three people gunned down at Jewish community center in Kansas in 2013; nine slain at a black church in South Carolina in 2015; and 11 killed at a Pittsburgh synagogue last year.

The Trump administration starkly acknowledged the problem in its October 2018 National Strategy for Counterterrorism.

"Notably, domestic terrorism in the United States is on the rise, with an increasing number of fatalities and violent nonlethal acts committed by domestic terrorists against people and property," the strategy paper says.

But the federal response has not accelerated to keep pace with the threat.

There are no designated domestic terror organizations

After the terrorist attacks of Sept. 11, 2001, the U.S. government re-oriented itself to fight the threat of Islamic terrorism, both inside and outside U.S. borders. Congress passed an authorization of military force against al Qaeda, and the State Department had already begun a process of designating foreign terrorist organizations.

Under federal law, anyone who provides so-called material support to a designated terrorist organization can be prosecuted.

Using this law, the Justice Department has convicted and imprisoned hundreds of Americans, often as a result of undercover sting operations. The investigations are relentless and the sentences are harsh; last year a former Washington metro transit officer was sentenced to 15 years in prison for sending a few hundred dollars in gift cards to ISIS.

Yet even as the threat from al Qaeda and ISIS has waned, the vast majority of U.S. government counterterrorism resources remain directed at international terrorism, according to congressional testimony.

Moreover, since there is no domestic terrorism statute, there is also no law against material support to domestic terrorism. And there are no designated domestic terrorism organizations.

The idea of designating any group in the U.S. a terrorist organization has long been seen as problematic on First Amendment grounds. Although Canada in June designated two violent white supremacist groups as "listed terrorist entities," legal experts say the U.S. would face constitutional challenges in taking that step.

"Somebody's going to have to make a determination about when a certain group crosses the line — you're getting into some really hairy constitutional questions," said Raymond Holcomb, a former FBI counterterrorism investigator.

Yet many proponents of a domestic terror statute say one could be enacted even without designating specific organizations as terror groups.

That's the view of Mary McCord, who was assistant attorney general for national security in the Obama administration.

At a basic level, she said, a domestic terrorism statute could provide an organizing principal to redirect government resources at the domestic terrorism problem.

"Right now they don't really have that mandate," she said. "No one has said, 'Throw everything you've got at this, the way you did for international terrorism.' It would help a lot to have a domestic terrorism offense because then they would be like, 'Here's this statute we are angling toward.'"

As it stands, suspected domestic terrorists are often investigated by the FBI's Civil Rights Program under the rubric of hate crimes.

A domestic terror statute would allow the FBI more latitude to monitor internet activity by people espousing racist violence, McCord said. As a practical matter, the FBI's counter terrorism division has more investigative resources. And terrorism analysis also conducted by a separate agency the National Counter Terrorism Center, whose analysts can sift through both international and domestic intelligence databases to map connections between extremists at home and abroad.

"It would allow intelligence analysts to look at expression by white supremacists that goes past just complaining about things but instead talked about doing something," said Nicholas Rasmussen, who headed the National Counterterrorism Center under Presidents Obama and Trump. "If you could identify individuals who are speaking an interacting with others in that action oriented way, that could trigger investigative scrutiny."

That's exactly what U.S. intelligence agencies have been doing for years against Americans who display allegiance to al Qaeda and ISIS. But the difference has been that those are designated foreign terror groups. Deciding when garden variety racism crosses the line into potential domestic terrorism may be a dicier proposition.

To be sure, it's far from clear any such intelligence gathering would have led authorities to the alleged El Paso shooter, who purchased his assault rifle legally and posted a hate-filled screed just minutes before his shooting spree. It remains to be seen whether he developed any relationships online with fellow white supremacists.

"I don't know if a domestic terrorism statute is a good idea," Rep. Jim Himes of Connecticut, a Democrat on the House Intelligence Committee, told NBC News. "But what I can tell you is that it is high time that we rebalance our investigative efforts in the direction of the things that are killing lots of Americans, which are white guys with guns."