Staff feared for their children's safety following threats.
A woman who sent death threats to staff of a drug testing firm has been jailed for six months. Animal rights campaigner Charlotte Lewis, 28, admitted four charges of harassment against existing and former staff of Huntingdon Life Sciences (HLS) in Cambridgeshire.
Lewis of Thornton Heath, Surrey, was linked to the offensive mail when saliva on the envelopes was found to match her DNA.
Charlotte Lewis has shown "remorse".
The letters began either "Dear Animal Abusing Scum" or "Dear Scum" and one warned: "If you don't quit HLS then your life will not be worth living. You will always have to be looking over your shoulder."
In the other Lewis wrote: "This is a warning. Your life is in grave danger if you don't stop working at HLS ...
"You will find yourself having a gun aimed at your stupid ugly head."
Peterborough Crown Court heard Lewis did not have "a violent bone in her body" and had no intention of carrying out the threats.
Sentencing Lewis, Judge Richard Pollard said the jail term would have been longer if Lewis had not pleaded guilty and shown remorse.
History of depression
He also issued a restraining order against Lewis, a member of Stop Animal Rights Cruelty, banning her from harassing HLS staff.
Lewis, who has a history of psychological problems and suffers depression, is expected to be released after three months on licence.
The judge said the "chilling letters" had brought "fear and anguish to people going about their peaceful and lawful business".
Lewis had written two letters which were sent to Mark and Julie Farrance, Charles Owen, and Jennifer Howlett.
Late-night demonstration
Mrs Howlett, who had not worked at HLS for two years, was left "hysterical and very scared", while Mr Owen was convinced an attempt would be made to carry out the threats.
The Farrances, who have received some 50 letters from animal rights activists, were also the targets of a late-night animal rights demonstration outside their home.
A brick was thrown through their window.
Shortly afterwards they received Lewis's letter in which she wrote: "I was there when a brick was put through your window. If you don't quit HLS you can expect more of the same."
Brian Cass says courts are taking threats seriously
Mrs Farrance feared for the safety of herself and her children and had to take time off work for stress.
Welcoming the sentence HLS chief executive Brian Cass said: "We respect people's right to protest but with that comes responsibility and it is good to see the process of law being upheld in this way."
Earlier this month HLS was saved from the brink of collapse by American financiers after the Royal Bank of Scotland demanded the repayment of a £20m loan.
The government is currently considering new police powers to clamp down on those protesting outside people's homes or sending malicious mail.