People involved in the initial import and spread of the rabbit-killing calicivirus disease have closed ranks and are keeping their mouths shut, the Agricultural Ministry said.

The man leading the investigation, the Ministry's enforcement unit national manager, Jockey Jemsen, said this was possibly because they wanted to use the same networks for another illegal importation.

The virus was smuggled into New Zealand last year and was spread at first on Central Otago farms. Though spreading the virus is now legal, the ministry could prosecute those who initially smuggled and spread it.

Mr Jensen said the investigation was still open, but not a lot of information had come to hand this year. "I guess everyone in the fraternity down there [in central Otago] have closed up shop on us and are reluctant to bother talking to us.