Why terrorism has become so hard to fight

The "bloodshed must end," President Donald Trump said of the latest deadly terrorist attack in London, but counterterrorism and law enforcement experts say efforts to curb the evolving threat of terrorism is complex and may never result in the complete eradication of such violence.

Trump's Sunday night remarks at Ford's Theatre in Washington, D.C., were the first he delivered about the Saturday attack apart from his previous tweets, and he used them to denounce terrorists, whom he called "a vile enemy," and also to offer solidarity to exasperated U.K. Prime Minister Theresa May, who herself has said "enough is enough."

But Alexander Meleagrou-Hitchens, the research director of the Program on Extremism at George Washington University, said preventing attacks in the modern era is challenging partly because of how the terrorist recruitment process has evolved in recent years, and the kind of people who are susceptible to terrorist propaganda.

"Stopping everyone who rents a van or buys a long-knife is really difficult," Meleagrou-Hitchens told ABC News. "Once that person is on the road, it's just very hard to stop them."

He noted that many terrorists and would-be terrorists may not commit a crime "until they carry out an act of mass murder," making it difficult for investigators to pick up on in advance.

Meleagrou-Hitchens cited the U.K.'s previous use of a "control order," which was a controversial policy that restricted the freedom of people believed to harbor terrorist inclinations.

Such measures, however, come at the cost of human liberty, which is why they were eliminated by the U.K. government for being too restrictive.

The Terrorism Prevention and Investigation Measures Act of 2011 (TPIM) abolished control orders in the U.K. and offered a new set of powers to allow the British home secretary to restrict the behavior of terrorist suspects in a way deemed more acceptable under human rights laws. The identity of those under TPIMs is typically kept a secret, according to the BBC.

But TPIMs, the orders given to monitor terrorist suspects after the abolition of control orders, are seldom used, the Telegraph newspaper reported in 2014.

Meleagrou-Hitchens also mentioned the possibility of intervening and counseling potential terrorists at the community level as a way to mitigate attacks, something he likened to "gang intervention."

John D. Cohen, an ABC News contributor and former U.S. counterterrorism coordinator, said Sunday that determining which people are more susceptible to ISIS's online propaganda campaign, and intervening, could be more effective than policing the spread of propaganda itself.

Meleagrou-Hitchens described such people as those who potentially have grown up in a moderate Muslim household without exposure to extreme ideology, and then latch onto ISIS propaganda because it "speaks to trauma in their lives, and their own direct experiences in the world."

But locating such individuals, and intervening in their lives, can be challenging, according to Meleagrou-Hitchens.

"It's not only that they were exposed to propaganda online," he said. "Why did they start searching for it in the first place? What caused them to want to seek it out?"

A big debate is looming on encryption

Prime Minister May has advocated regulating cyberspace in the wake of the attacks which, like Trump, she said she wants to stop.

"It is time to say enough is enough," May said of what she described has been "far too much tolerance of extremism" in the U.K.

Steve Gomez, an ABC News contributor and former FBI agent, said a big debate is on the horizon regarding how to deal with encrypted apps, which enable propaganda to be spread beyond the eyes of law enforcement officials.

"Law enforcement often can't conduct a wiretap on a suspect because of encryption," Gomez said.

Ending modern-era attacks altogether is unrealistic, he said, though they could be mitigated with new legislation that tackles the encryption issue.

Privacy advocates, who are concerned about government gaining access to information about our personal lives, typically support the availability of strong encryption.

Gomez also noted that the evolving terrorist M.O. does not require would-be terrorists to get in touch with any kind of central command or ISIS authority prior to committing an act of violence, adding an additional layer of difficulty in cracking down on communications that lead to terror.

"ISIS has planted seeds and has been very good at putting out messages to potential followers online to go out and kill," Gomez said. "Then, after the fact, they will claim the attacker as one of their soldiers."