( CNN ) New Zealand police have revealed a threat to contaminate infant formula and other dairy formula with poison , in an apparent attempt to blackmail the government over its pest control policies . Police appealed for public help to find the alleged blackmailer , at a press conference in Wellington Tuesday . They revealed an investigation had been underway since November , when anonymous letters were sent to giant dairy cooperative Fonterra and a farmers ' lobby group . The letters were accompanied by small packages of milk powder containing a concentrated form of 1080 ( sodium fluoroacetate ) , a poison used by New Zealand 's Department of Conservation to control introduced pest species such as possums and rats . The letters threatened to contaminate infant and other formula with 1080 unless New Zealand stopped using the poison for pest control by the end of March , police said in a statement . The letters said the threat would be made public if the demand was not met . New Zealand 's heavy use of 1080 -- it uses about 80 % of the world 's manufactured stocks , according the country 's Environmental Protection Agency -- has been a controversial issue , with conservationists and farmers generally supporting the measure , and some hunters and animal rights activists opposed . Primary Industries Minister Nathan Guy said the threat amounted to `` eco-terrorism . '' Prime Minister John Key said at a press conference that police had advised that there was a low likelihood of the threat being carried out . Meanwhile , government officials said supplies of formula had not been contaminated , and urged consumers to continue using the products . `` We are confident that New Zealand infant and other formula is just as safe today as it was before this threat was made , '' said Scott Gallacher , Deputy Director-General of the Ministry for Primary Industries . `` People should keep using it as they always have . '' He said more than 40,000 samples had been taken without finding a trace of 1080 , and food security measures had also been boosted in response . These included a strengthened security measures in retail stores and a boosted presence at manufacturing sites , and enhanced milk and product testing , including the introduction of a test for 1080 . `` This criminal threat is designed to cause fear in order to generate a political outcome . It is using food as a vehicle but should not undermine confidence in our world-class food safety system or in any manufacturer , '' he said . He urged consumers to check formula packaging for any signs of tampering as a result of the threat . Police said an Auckland-based investigation team was pursuing a number of lines of inquiry in locating the source of the threat , but the time was right to appeal to the public . `` Whilst there is a possibility that this threat is a hoax , we must treat the threat seriously and a priority investigation is underway , '' said Deputy Commissioner Mike Clement . `` You might be aware of someone who has strong views on the 1080 issue and made threats , or has discussed how to access supplies of 1080 . '' He also called on the alleged blackmailer to come forward . `` The letter writer may not have really considered the implications of their actions when this communication was drafted . Now is the time to put this right by picking up the phone and calling us . '' Blackmail carries a maximum sentence of 14 years jail in New Zealand . New Zealand is the world 's leading dairy exporter , and dairy cooperative Fonterra is the country 's biggest company , with revenue of $ 16.3 billion last year . CEO Theo Spierings described the threat in a press conference as `` a despicable crime . '' The global diary giant has been beset by problems before , most recently in 2013 , when it ordered a global recall of products -- including infant formula -- after fears they might contain bacteria that could cause botulism . The fears proved unfounded , as the bacteria detected was not a botulism-causing strain . New Zealand Infant Formula Exporters Association chairman Michael Barnett said eyes were now on the response to the threat in key markets such as China , the biggest consumer of New Zealand dairy exports . `` This could be extremely damaging for New Zealand 's exports offshore , '' he told CNN affiliate TVNZ . `` Our whole reputation as an exporter of food is at risk because of this nutter . '' Trading in all New Zealand-listed dairy companies was halted Tuesday with the announcement of the threat . CNN 's Charles Riley contributed to this report .
New Zealand police reveal threat to poison infant formula . Letters were sent containing contaminated formula and a demand to government . Police say they are investigating the incident as an act of terrorism.