Dr Kirker said the letter-writer would not necessarily have a history of poor mental health, or be "outwardly mad", but was likely to be on the fringe of society and spending a lot of time inside their own head.
"The type of person who would go through with it would actually be deranged. Where it seems this is more the type whose aim is to manipulate to get what they want. The ones who would go through with it would want people to get hurt. This is just a dramatic way to make a point."
Former police negotiator Lance Burdett said in most cases people the police dealt with were criminal, suicidal or had mental illness. Those behind the threats made public yesterday sat above those groups, in a class populated by terrorists and zealots, he said.
"That group are usually intelligent, that's what makes them hard for police to find. They know how to cover their tracks," Mr Burdett said.
"Generally people with a mental disorder don't know how to keep themselves secret."
Mr Burdett said zealots could be particularly difficult to deal with from a negotiator's perspective.
"They have a radicalised mindset. They can't be convinced otherwise. They're the types who chain themselves to buildings and you have to eventually cut the padlock rather than talk them down."
In New Zealand, the extent of radicalism wasn't usually very severe, and its purpose was mainly to scare.
"They do whatever they want, play with your brain and work on fear.
"But they are unlikely to carry threats out."
Mr Burdett felt police would have only come forward after being unable to find the perpetrator on their own.
