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Stopping the kind of vehicular attacks like the one that killed eight people in New York City on Tuesday afternoon has been a top concern for law enforcement agencies worldwide in the wake of deadly attacks in the United Kingdom, France, Germany and now the U.S.

But experts say it's also one of the hardest threats to predict and guard against.

"This kind of attack is very difficult, and probably the most difficult, for law enforcement to protect and prevent," said Steve Gomez, former FBI agent and ABC News contributor.