Why was London attacker under surveilance?

Sudesh Amman is understood to have been regarded by security services as an 'extremely concerning individual'.

He was under 24-hour police surveillance since his release from prison less than a fortnight ago and monitoring is thought to have intensified in light of growing concerns about his behaviour.

But how do authorities decide who needs to be watched?

Richard Walton, a senior fellow for think tank the Policy Exchange and former head of the Metropolitan Police's counter terrorism command, answered questions from the PA news agency.

What level of threat would someone like Sudesh Amman pose to warrant 24-hour surveillance?

The justification for authorising a surveillance operation of this scale would require intelligence that the offender posed a substantial risk to the public.

If surveillance officers consider that a subject who is under surveillance is about to commit a violent act, they will always intervene to protect the public

The officers were armed and able to deal with any threat posed by the suspect. The armed officers shot dead the suspect within 60 seconds of him attacking the members of the public with the knife.

What level of resources does an operation like that take?

The number of officers involved in a covert surveillance operation will vary depending on a number of factors, including the threat the suspect poses and their lifestyle.

Typically how much would that cost?

Surveillance operations are resource intensive and expensive, and require the deployment of highly-trained officers.

If sustained for long periods of time, they can cost tens or even hundreds of thousands of pounds.

In the three latest attacks all the attackers have worn fake suicide vests. What does this tell you about the mindset of the attackers?

Islamist terrorists wear fake suicide vests for various reasons: to create fear and panic in the public and in order to try to encourage police to shoot them so that they can become what they consider to be a 'martyr'.

Police officers will always try and arrest a suspect if possible but it is not always practicable.

What do counter-terrorism officers need to consider and take into account when they see someone wearing such a device?

It is impossible to know whether a 'suicide vest' is a genuine bomb or a fake vest.

Officers will always seek to protect the public and respond to the threats in front of them.

They will always attempt to keep the public away from someone who is wearing such a vest.

If an officer is faced with a threat to life, they will take action to protect the public by using their firearm.