Only a few people had authorised access to the 1080 poison most likely used in the baby-milk blackmail letters, says a food safety expert.
The letters Fonterra and Federated Farmers received in November contained white powder, not the coloured, dyed pellets containing additives used in bait drops.
Yesterday, police would not rule out the chances that a blackmailer had collected dropped 1080 pellets and managed to refine them back to white powder.
But that theory sparked a sceptical response from Massey University food safety specialist Dr Steve Flint.
"That's highly unlikely. They'd have to have a knowledge of chemistry and facilities to do that. That's certainly not a sensible approach."
Laboratories or suppliers were the most likely source, Dr Flint said.
Meanwhile, security has been ramped up at supermarkets and laboratories in the wake of the threat.
Environment Minister Nick Smith said current controls on 1080 poison were "robust", but "additional controls" would be enforced in laboratories.
The anonymous letters said infant formula would be contaminated with 1080 if New Zealand didn't stop using the poison by the end of March.
Current laws had an exemption for research laboratory use, as was the case for dozens of similarly toxic substances, Dr Smith said. The Government had discussed tighter security with research laboratories and the Governor-General signed off the new regulations on Tuesday night.
The quantity of the poison that was being stored and used would be tracked and importers would need to obtain an Environmental Protection Authority certificate.
The new rules would make it unlawful for anybody, including research laboratories, to possess 1080 without EPA approval. "It will enable the authority to better track the importation, distribution and use of high purity 1080, and ensure it is always securely contained," Dr Smith said.
Prime Minister John Key said China had kept its borders open to New Zealand, but set some conditions on the importing of infant and other formula. He defended making the threat public, saying not going public would have been seen as a "cover-up".
Food and Grocery Council chief executive Katherine Rich said she was briefed by the Ministry for Primary Industries (MPI) in mid-February, and supermarkets "immediately kicked into gear".
Foodstuffs had set up "milk monitors", staff who watched infant formula and could give customers MPI advice, additional CCTV cameras and signage in all supermarkets.
All infant formula had been removed from the shelves of The Warehouse's 92 stores nationwide and could be purchased from the customer service counter. CCTV cameras would remain on the product at all times while in storage and when behind the counter.
A staff member was monitoring the infant formula area at New World Ilam in Christchurch yesterday.
The shelves were cordoned by red-and-white tape and information leaflets and signs were highly visible. Shoppers were limited to two tins each.
Nearby Pak'nSave Riccarton had removed its infant formula from the shelves.
They were now being kept in a "secure location" with 24-hour CCTV surveillance.
Deputy Commissioner Mike Clement said police had "many persons of interest" in the inquiry and police expected to gain more leads now the threat had been made public.
It emerged yesterday that police had been contacting known 1080 opponents since the threat was revealed.
Ban1080 party candidate Tricia Cheel said detectives visited her on Tuesday night.
"I guess they were just following up and wanted to check whether I knew anybody that may be capable of doing such a thing," she said.
"I felt that surely they were looking in the wrong place."
Ms Cheel said she had been campaigning for 25 years to get 1080 drops stopped and saw the practice as "eco-terrorism against the community".
High-profile anti-1080 campaigner Clyde Graf was contacted by police yesterday. They had contacted another South Island campaigner too.
Mary Molloy of Farmers Against Ten Eighty was yet to be contacted. But she believed the powder was sent from someone "in the higher-up levels of the poison brigade".