Watching potential terrorists is anything but an exact science and police can't follow everyone
The attack in London on Wednesday shocked even the most experienced counter-terrorism officers. You always know the stakes are high, you know that many innocent people will be killed if an attack gets through and you work tirelessly to prevent it.
It is still deeply shocking when it happens. As the head of the Metropolitan Police Counter Terrorism Command, I oversaw the response to the murder of Drummer Lee Rigby. His killers had previously been deemed to be low-level extremists who were not an imminent threat.
Theresa May has revealed that the attacker at Parliament Square had similarly been viewed as a "peripheral figure" by MI5. But that does not mean police and intelligence agencies took their eye off the ball.