WASHINGTON • Following two mass shootings at the weekend, US President Donald Trump on Monday called on the federal authorities to do a better job identifying violent extremists in the US.
But that will not be easy.
Federal investigators looking to prevent acts of domestic terrorism, like the massacre of 22 people at a shopping centre in El Paso last Saturday, have fewer tools and authorities at their disposal than they would if they were up against someone tied to an international organisation such as the Islamic State in Iraq and Syria (ISIS) or Al-Qaeda.
That challenge has revived questions about whether the Federal Bureau of Investigation, which transformed itself after the Sept 11 attacks to combat international terrorism and acquired broad new surveillance powers, is adequately positioned to confront a white nationalist threat responsible for some of the deadliest acts of violence in the last few years.
Former FBI counterterrorism supervisor David Gomez said: "I can go online and say whatever I want, but that doesn't mean it's sufficient for the FBI to open an investigation. You need to combine the free speech with an overt act, and that overt act has to be something criminal in nature."
The laws, as they exist, "are not designed around the FBI being able to prevent these actions", Mr Gomez added. "The laws are designed to respond to crimes already committed and then investigate them."
Confronting domestic terrorism is an urgent issue for law enforcement at a time when white supremacists and like-minded extremists are causing more murders - including a rampage at a Pittsburgh synagogue last October that killed 11 people - than Americans inspired by foreign groups.
The FBI made about 90 domestic terrorism arrests in the first three-quarters of the year and has hundreds of open cases.
Still, Mr Trump said law enforcement "must do a better job of identifying and acting on early warning signs".
"I am directing the Department of Justice to work in partnership with local, state and federal agencies, as well as social media companies, to develop tools that can detect mass shooters before they strike," the President said.
That is easier said than done, with part of the challenge arising from how federal law distinguishes between international terrorism and domestic terrorism.
The authorities conducting international terrorism investigations, for instance, can get a secret surveillance warrant to monitor the communications of a person they think may be an agent of a foreign power or terror group.
Similarly, the United States criminal code makes it a crime for anyone to lend material support to designated foreign terror organisations, including ISIS and Al-Qaeda, even if the accusation does not involve accusations of violence.
There is no domestic counterpart to that material support statute, meaning federal prosecutors must use weapon laws, hate crime laws and other approaches. Mere membership in, or support for, a white supremacist organisation is not illegal.
And decades after accusations of surveillance abuses in the era of former director J. Edgar Hoover, FBI officials consider themselves duty-bound to follow internal guidelines meant to respect free speech.
Other obstacles include whether technology companies can adequately flag troublesome behaviour in advance, and whether law enforcement can successfully separate out those bent on violence from those who simply mouth off about it.
The perpetrators of extremist attacks often act by themselves without any affiliation to a broader movement or organisation, which can thwart efforts to identify them beforehand.