Mourners at a memorial outside of the Cielo Vista Walmart.

WASHINGTON — The mass shooting at an El Paso Walmart and arrest of a man whose white supremacist manifesto railed against a “Hispanic invasion of Texas” have heightened calls for Congress to enact a federal domestic terrorism law.

In a statement last week, the president of the FBI Agents Association, Brian O’Hare, urged Congress “to make domestic terrorism a federal crime. This would ensure that FBI agents and prosecutors have the best tools to fight domestic terrorism.”

It is not clear, however, whether such a statute would make a practical difference in what the government can already do under existing law.

Here is a breakdown of the legal policy issues surrounding domestic terrorism.

What is the legal difference between domestic and

international terrorism?

A federal law defines terrorism as crimes of violence that are intended to intimidate or coerce a civilian population or government policy. But it distinguishes between “international” terrorism, which must have a foreign or transnational nexus, and “domestic” terrorism, which occurs primarily on American soil.

“Acts of terrorism transcending national boundaries” is a federal crime, giving the FBI and federal prosecutors jurisdiction to take the lead. There is no equivalent crime of domestic terrorism, so law enforcement officials deal with such offenses using other laws that do not have “terrorism” in their labels, like the state-level crime of murder.

What difference does that make after an attack?

If a domestic terrorist survives and is prosecuted, federal officials can still sometimes assert jurisdiction. Timothy McVeigh was prosecuted in federal court for the 1995 Oklahoma City bombing under a federal law that bars “weapons of mass destruction,” for example.

But terrorist attacks involving guns are likely to be handled differently if a terrorist’s ideology cannot be tied to a foreign power. Dylann Roof, who in 2015 killed nine African American churchgoers in Charleston, South Carolina, for example, was charged with murder in a state court.

Sometimes both systems can be used. Robert Bowers has been charged with federal hate crimes for the 2018 attack on a Pittsburgh synagogue, but he also faces state murder charges.

Is this difference

substantive or symbolic?

The fact that under existing law, either type of terrorist will end up serving a long sentence or facing execution raises the question of whether creating a new federal crime of “domestic terrorism” would make any meaningful difference after an attack — beyond determining which set of prosecutors handle a big case — or would instead be largely symbolic.

But Mary McCord, a former senior Justice Department national security official who has long called for enacting a domestic terrorism law, suggested that the symbolic element could make a substantive difference to the country.

Among other things, she said that the government needs to maintain trust with Muslim-American communities so people will sound warnings if they hear something potentially dangerous. Calling Islamist attacks “terrorism” in court, but not doing the same for white supremacist attacks, is a racist double standard that undermines such trust, she argued.

Would changing the law help prevent attacks?

Federal law enforcement officials can seek to imprison people who “provide material support” to foreign terrorist groups — like sending them money or trying to join them — without the risk of waiting to see if they develop plans to personally carry out attacks. Could Congress broadly extend this system to jail people for helping domestic extremist groups?

Probably not, said Cole, who helped litigate two major cases on the scope and limits of material-support laws. He said the Supreme Court would likely rule that the First Amendment bars the government from making it a crime to provide otherwise lawful support to a domestic organization.

In the first case, a group of Americans challenged the use of a material support law to bar them from providing otherwise lawful legal training to Kurdish and Tamil groups that the government had designated as terrorists. The Supreme Court in 2010 sided against his clients, but its reasoning heavily stressed the foreign nature of the targeted groups.

“We also do not suggest that Congress could extend the same prohibition on material support at issue here to domestic organizations,” the majority opinion said.

Indeed, in the other case, an appeals court ruled in 2011 that the First Amendment barred charging Americans with material support for providing otherwise lawful help to an Oregon-based charity that the government had deemed a terrorist organization for sending funds to foreign terrorists. The ruling emphasized the charity’s domestic ties in reaching that result.