How to measure the terrorism threat after the Westminster attack

Under the UK's system, the country can never be entirely free from the threat of terrorism

Two questions have been particularly prominent in the aftermath of the attack on Westminster. First, why was the attacker – named by police as Khalid Masood – not apprehended, given that he appears to have been already known to police and intelligence services. Second, what does this attack mean for security in London and elsewhere.

The UK’s counter-terrorism architecture involves a five-tiered warning system organised around five levels of threat: low, moderate, substantial, severe and critical. It is similar, in this sense, to the colour-coded warnings of the US Homeland Security Advisory