War on terror

How Do You Stop a Future Terrorist When the Only Evidence Is a Thought?

Across Europe and the United States, law enforcement officials are struggling to reckon with attackers like Mr. Abballa and Omar Mateen, whose shooting rampage this month at a gay nightclub in Orlando, Fla., left 49 dead. They are men who clearly seemed to be building toward violent acts, and whose names had surfaced in terrorism investigations, but who avoided crossing legal lines that could tip off the authorities until it was too late.

With thousands of terrorism surveillance cases running at any given time, the European authorities say they are swamped and are in the difficult position of trying to head off attacks of which the only forewarning is often in the form of what someone thinks or what they are overheard saying.