A man has been found guilty of sending a "shocking and offensive" package to Huntingdon Life Sciences.
A jury at Birmingham Crown Court took three hours to find Andrew Kirk, 46, of no fixed address, guilty of sending an obscene or indecent postal package.
Kirk, who was cleared of 17 other counts of the same offence, was given conditional bail until December.
The court heard he also sent packages to senior prosecutors in Durham, Humberside and Staffordshire and an MP.
A total of nine packages containing insults and graphic images of animals were sent.
The MP for Huntingdon, Jonathan Djanogly, was another recipient.
The jury accepted Kirk's argument that none of the letters was indecent or obscene, but decided that an envelope emblazoned with a swastika in which one of the letters was sent had been in breach of the law.
It had been mailed to Huntingdon Life Science's site at Wooley Lane, Huntingdon, and was addressed to the "sons and daughters of Josef Mengele" and branded the facility an "Auschwitz Laboratory".
Kirk denied all 18 counts of sending obscene or indecent packages through the postal system.
The 46-year-old, formerly of Swanage Close, Middlesbrough, accepted he had sent the items when he was interviewed by police earlier this year.
Steven Bailey, opening the case for the prosecution, told the jury: "He thought that the 'extermination' of animals in the lab was akin to the Holocaust."
Kirk will be sentenced in December.