-- British Prime Minister Theresa May responded strongly to the attack in London that killed seven and left dozens injured, saying in a statement that "things need to change" and calling for among other things an international agreement to crack down on the online presence of the ideology of Islamist extremism.
But two counterterrorism experts told ABC News that trying to eliminate the presence of terrorists' ideology online would be difficult. Saturday night's attack, the third in Britain in three months, began on London Bridge when a van plowed into crowds of people and continued in nearby Borough Market with three knife-wielding men jumping out of the vehicle and stabbing people apparently at random.
"We cannot allow this ideology the safe space it needs to breed," May said in a statement Sunday. "Yet that is precisely what the internet – and the big companies that provide internet-based services – provide. We need to work with allied, democratic governments to reach international agreements that regulate cyberspace to prevent the spread of extremism and terrorist planning. And we need to do everything we can at home to reduce the risks of extremism online."
John D. Cohen, an ABC News contributor and former U.S. counterterrorism coordinator, said, "It's very difficult to control cyberspace and social media."
"It would be more effective to better understand why a growing number of people are inspired by what they view [online] and address the underlying cause," he said. "This is where law enforcement is focusing their attention now."
Certain people are more likely to be inspired to commit acts of violence after being exposed to propaganda, and reaching out to them is critical to preventing lone-wolf terror attacks, he said.
"These are typically people who live in the West and feel disconnected from other people," Cohen said. "They may have encountered a series of failures in their lives and want to connect to a cause that gives their life meaning."
"These are not necessarily people who have a deep connection to religion," he said, despite the propaganda of groups like ISIS being cloaked in religious rhetoric.
He added that efforts to find susceptible people and intervene are already being explored both in the U.S. and Europe.
"It's very difficult," Kelly said. "We would need greater cooperation from tech companies."
Kelly said the highest priority in preventing terrorism in big cities such as New York is to gather intelligence.
He added that he does not believe the United Kingdom is as advanced as the U.S. yet in intelligence gathering on terrorist plots although he said the two countries frequently share information.
Once investigators know of possible terrorist plots, "a door-knocking campaign" is sometimes effective as a way to speak to potential suspects face to face. In other instances, he said, intense surveillance is needed, an effort that he acknowledged frequently requires a lot of manpower.
"It's not always easy to make the right judgment about potential suspects because sometimes they can seem normal," Kelly said. "That's why intelligence is the key."