When anonymous letters threatening to spike infant formula with 1080 were delivered to Fonterra and Federated Farmers, there wasn't much evidence for police to go on. Two letters, one envelope (the other was not kept) and milk powder laced with poison.

"It was a needle in a haystack," said Detective Superintendent Andy Lovelock, the officer in charge of finding who was responsible.

"A needle in a haystack and frankly we didn't know whether we were in the right haystack. And there were a couple of haystacks which didn't work out either."

While the Ministry of Primary Industries moved quickly to secure the safety of the dairy supply chain, testing 150,000 samples, the blackmail letters also triggered an unprecedented police investigation.

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Ultimately, Operation Concord cost taxpayers $4.85 million and led to the conviction of Jeremy Hamish Kerr. The 60-year-old was sentenced today to eight and half years in prison, where Justice Geoffrey Venning said his actions were "near the most serious case" of their kind.

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The letters were sent in November 2014, demanding the Government stop using 1080 by March 27 the following year or else "several New Zealand infant and other formula will be released into the retail chain in the Chinese market and one other market with traces of 1080".

There was no DNA or other forensic traces on the documents and the language used in the letters - "Our group has no confidence in the political or democratic process concerning 1080" - purported to be a disaffected environmentalist.

The threat was real, no hoax; testing showed there was enough poison in the posted samples to kill between 13 and 33 infants, depending on their weight.

"If you ever needed your mind focused, that was it," said Lovelock, "It wasn't like we could take a gamble on whether they had 1080 or not."

Behind a closed door in Harlech House, a grey building in the middle of Otahuhu where many of the most covert police investigations take place, a team of 35 detectives and analysts were drawn together to find the needle in the haystack.

As the March deadline drew closer, the public were told about the threat. Prime Minister John Key condemned it as eco-terrorism. The Ministry of Primary Industries reassured parents the supply chain was safe and tests of 150,000 samples showed no traces of 1080. Supermarkets pulled baby milk powder off the shelves. And the police urged the blackmailer to come forward.

"For months and months and months, our key message was to encourage that person to come forward," said Lovelock.

"On the basis that they've sent these letters and regretted it, come forward and we can sort this out."

But Jeremy Kerr never did.

With Operation Concord now able to move freely without fear of the 1080 threat leaking, police started working through the 2600 persons of interest.

Using a weighting system to screen the risk of each suspect, police whittled the list down to 100 over a couple of months, then 60. Each of those were interviewed by police and asked to provide a DNA sample, fingerprints and a preliminary search of their computer.
As it turned out, Kerr agreed to the request, seemingly confident enough he had left no trace.

For months, the police believed the letters were sent through a South Island mail centre and focussed the investigation there. This was one of the wrong haystacks.

But detailed analysis of the post mark on the Fonterra threat letter, dated 24 November 2014, police managed to track down the individual who actually made the date stamp.

"What was helpful to us was the person had never made a stamp for the South Island," said Lovelock. "Which meant we had to rethink what we thought we knew."

The focus of Operation Concord switched to the North Island, as police now knew the letter was processed through the Wellington mail catchment area. Kerr later confessed to police he posted the letters from Paraparaumu. "So we were right," said Lovelock.

Struck to that door in Harlech House is a picture of a combine harvester; Operation Concord sucked up every scrap of information to sort the wheat from the chaff.

They drew up a list of 2600 suspects. Anyone and everyone who had ever expressed an opinion on 1080, adverse or favourable, as well as those who had lawful access to the poison.

This was because the 1080 contained in the threat sample was in a very pure form, not a crushed green pellet picked up in the bush.

Jeremy Kerr was on the list because of his involvement in the poison industry since the mid 1990s. He had also access to 1080 and was involved in research with Government agency Landcare. His possum control poison Feratox was a rival to 1080 - he had once earned up to $250,000 a year in royalties but this had fallen away to less than $100,000 in recent years.

Working closely with ESR scientists, the police worked their way through the list of those who had lawful access to the poison.

Using a scientific process which measured carbon, hydrogen and oxygen atoms, the police were able to eliminate the majority of the 1080 samples submitted for analysis. A cluster were similar in chemical structure to the blackmail sample - Jeremy Kerr had previously supplied two of those samples to Landcare.

"This wasn't done in five minutes," said Lovelock. "It took months and months. It's very interesting science which we're very pleased about."

The 'retraction letter' Jeremy Kerr sent to Detective Andre de Villiers. DNA which was 260 times more likely to be Kerr's was found on the letter.
The 'retraction letter' Jeremy Kerr sent to Detective Andre de Villiers. DNA which was 260 times more likely to be Kerr's was found on the letter.

The crucial breakthrough came in July 2015.

When Kerr agreed to provide DNA and fingerprints to the police, he was dealing with Detective Andre De Villiers who was seconded to the investigation from his beat in Gore.

Kerr, who had access to 1080 for research purposes dating to the 1990s, told the detective he had got rid of the 1080 he had, tipping about a kilogram down the toilet.

The detective left a business card and following this meeting, Kerr decided to write to De Villiers anonymously, claiming that there was never a threat to New Zealanders.

This "retraction letter" was sent to De Villiers at his home station in Gore - alerting police to the likelihood whomever sent the letter had previously spoken with the police officer in question. Kerr was on that list.

He also slipped up. Kerr left traces of DNA 260 times more likely to come from him, or male relatives, than any random person on the street.

A second "retraction" letter, which was unsent, was later found crumpled in a business linked to Kerr.

The 2600 suspects had been whittled down to one: Jeremy Hamish Kerr, Person of Interest number 1931.

A second 'retraction letter' was found in a business address connected to Jeremy Kerr. Photo / Supplied
A second 'retraction letter' was found in a business address connected to Jeremy Kerr. Photo / Supplied

Nearly a year after the 1080 letters were sent, the police raided Jeremy Kerr's home and business in east Auckland. He was interviewed by Detective Senior Sergeant Aaron Pascoe back at Harlech House, where he slowly but surely dug himself into a deep hole.

He told lie after lie until confronted by Pascoe with evidence to the contrary.

At 9.47am on 13 October 2015, the detective tells Kerr "now's the time to actually tell the truth because not telling the truth and us slowly identifying each area where you're not being honest isn't doing you any favours."

"No," said Kerr, "fair enough." And with that, he coughed to the crimes and said he was sorry.

He was arrested and charged with two counts of blackmail, pleading guilty at a hearing in December. Despite the early admissions, the 60-year-old steadfastly stuck to the claim he was motivated by "altruistic opposition" to aerial dropping of 1080. This was dismissed by Justice Venning at a disputed facts hearing, who ruled Kerr was motivated by money. He was under financial pressure and his royalties from sales of Feratox would increase if 1080 was banned.

This benefit was difficult to quantify but would be "relatively modest", said Justice Venning.

Compare this to the estimated $37 million cost to the country including more than $20 million attributed to Fonterra, and nearly $5 million for Operation Conchord.

"To my mind, this makes his offending even more reprehensible," said Lovelock.

"His potential financial gain, in the scheme of things, was pocket change [compared to] jeopardising an enormously lucrative market for New Zealand."

Now the sentencing is over, Operation Concord is winding up. Detectives will return to normal duties, files will be packed up and diagrams covering the walls of the office will come down.

One was the ABC list which Lovelock swears by.

Assume nothing. Believe no one. Check everything.