How curbing terrorist attacks has evolved into such a challenging ordeal

Experts suggest that the violence is here to stay.

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."

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.

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.

"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."