So why is the UK so bad at keeping tabs on terror suspects?
Business Insider has noted before that the job is more complicated than it sounds, even after the suspect has been identified. In France, for instance, former French intelligence counterterrorism chief Louis Caprioli estimated that it takes 18 to 20 officers to keep an eye, 24 hours a day, on any one suspect.
There are 6,000 employees at GCHQ and 4,000 at MI5. But there are up to 3,000 suspects in the UK. At the French ratio, you would need 60,000 officers to track them all. That’s almost half of Britain’s total number of police officers, 127,000 (PDF).