Last night a police spokesman confirmed that letter drop was one of a number of historic 1080-related incidents that the team was aware of.
"These matters have been or are being assessed to determine their relevance to the current investigation," he said.
"Police are fully focussed on the current investigation and we will not be drawn into discussion about previous events which may or may not be relevant to Operation Concord. Police will be talking to a range of people who may have information to provide.
"As is normal for any police investigation we will not go into any specific details about who these people are, or why we might want to speak to particular individuals."
However former investigators spoken to by the Herald gave insight into how police go about catching an anonymous threat-maker.
"There will be two parts to it. Initially there would have been a really close examination in terms of forensic evidence around the material that was received - the handwriting, the envelope, where the content was sourced from, whether there was any DNA or fingerprint evidence," said one ex cop.
"Then there will be an intelligence phase. They will be profiling and trying to narrow down their suspect list - given it's been four months since they started investigated they may now have a reasonably narrow focus. It would be unusual for someone to do this having not done something overt before."
He said going public was another tactic.
"People talk. Without any publicity they probably haven't had the opportunity for people to suggest suspects."
A former high-ranking investigator told the Herald the Operation Concord team would be "delving through records of disaffected people" and looking at those who have done this sort of thing in the past.
"They will visit 1080 protesters and speak to them to see if they have any information," he said.
"This is an extreme threat. Sometimes, single-issue nutters come out and they haven't offended before. But in this case I'd say the offender has been involved in some sort of protest action before."
He said police would treat the investigation like any other.
"They have a robust investigation process which in most cases, does identify who has committed particular offences. In this case they will be using all of those processes and all their investigation techniques as they would in any other case.
"I am sure that after this amount of time there are people that they are looking at very closely and it is more likely than not that they will (make an arrest)."
Another former investigator shed light on why police waited so long to release investigation details.
"The threat was never intended to come into effect until after March 2015, so the investigating agencies wouldn't want to cause drama if they had a window of time in which to investigate that would allow things to progress under the radar of the offender - otherwise they might get rid of evidence."
That was why police didn't interview the known 1080 protest group members before Tuesday.
"As soon as you do that you've lost control of the element of surprise. Police could undertake other investigative activities in relation to those persons, but if they're interviewed before the public release, then they're bound to tell friends and colleagues, then its out and work underway in the back ground could be compromised."
He described the culprit as "a very disaffected person" but "not necessarily brainless".
"After-all, this is a simple but highly effective way to hold a country to ransom" he said.
"Others who know this person will recognise the discontentment (they) may express and be able to link it to a specific issue like this.
"What's needed is for those associates to come forward, it isn't a time for misguided loyalties. Even the offender would get a degree of public respect by 'outing' themselves now, apologising and assuring everyone there was no risk anymore, - even though they would still likely face a jail term."
Anyone with information about the 1080 threat should call the Operation Concord team on 0800 723 665 or email opconcord@police.govt.nz. To pass on information anonymously, call Crimestoppers: 0800 555 111.
Poisoning more personal than political
Massey University forensic psychologist Dr Mei Williams gave further insight into the perpetrator of the 1080 threat, saying he or she might be a grieving pet owner.
"The threat appears to be more personal than political ... " she said. "They may have experienced a beloved animal die in severe pain from 1080 poisoning - hence the anger towards the authorities and the desire for revenge. For this reason, I don't see it as a group but rather an individualised act, and in some ways has a similar feel of extremist groups believing the end justifies the means.
"The person, or people, see their relationship with animals as more important than human relationships - hence the threat of harm to babies through poisoning of formula." Dr Williams said the offender may feel satisfaction at having created a sense of fear among the public.
