- guardian.co.uk, Thursday February 1 2001 02.28 GMT
The device went off as a volunteer at the British Heart Foundation shop in Penrith, Cumbria, opened the morning post. The woman was shocked but uninjured, police said.
It is the tenth device sent to premises in northern England and Wales and comes amid warnings from the home secretary, Jack Straw, that the government intends to crack down on intimidation from animal rights protesters.
The incident comes as a judge at Peterborough crown court jailed an animal rights activist for six months after she admitted sending threatening letters to employees of the animal research company Huntingdon Life Sciences.
Charlotte Lewis, 28, of Thornton Heath, Surrey, admitted four charges of harassment against current and former employees of HLS. The court heard how unemployed Lewis was arrested after forensic scientists matched her DNA to saliva on the back of stamps she had placed on envelopes.
Christopher Morgan, for the prosecution, said that Lewis had written two letters which began with the words "Dear Animal Abusing Scum" and "Dear Scum".
One letter 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."
The other said: "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."
In mitigation, David Heraghgy, defending, said the animal rights campaigner did not have "a violent bone in her body" and had no intention of carrying out the threats.
But Judge Pollard said Lewis's crimes were so serious and the letters had had such an effect on the recipients that a jail sentence was necessary. He said Lewis would serve three months in prison and then be released on licence.
A spokesman for Cumbria police said of yesterday's nail bomb that no one had claimed responsibility for the incident but they believed it, and other recent attacks, to be the work of animal rights extremists.
The police spokesman said: "What we can say is we are aware, because of our liaison with other forces, that previous incidents were perpetrated against people with connections to the agricultural animal industry."
The tactics employed by animal rights activists have changed since the early 1990s, moving away from isolated incidents towards targeted campaigns of direct action.
A number of small separate cells are believed to use the internet to coordinate their activities. It is not known if these cells are involved in the recent nail bomb attacks.
Robin Webb, spokesman for the Animal Liberation Front, has consistently denied that his organisation has had any role in the recent parcel bomb incidents.
Police are taking the attacks so seriously they are coordinating efforts by linking incident rooms across the country - only the third investigation of its kind in history. Nine forces are now involved in the investigation. The inquiry, called Operation Package is being coordinated by North Yorkshire police and led by the deputy chief constable, Peter Walker.
Yesterday's explosion came just one day after the discovery of two nail bombs which were safely defused by the army.
One of the packages was sent through the post to agricultural suppliers on the outskirts of Sheffield. The second was sent to a Cancer Research Campaign shop in Lancashire.
In December last year a six-year-old girl was injured when she opened a package at her father's pest control business in Congleton, Cheshire.
In another incident earlier this month a woman received severe facial injuries when a device sent to an estate agents in Patrington, East Yorkshire, exploded as she opened the envelope it was concealed in.
Letter bombs have also been sent to other businesses including a chip shop, a pet shop and a farmer in Ripon, North Yorkshire, at the beginning of this year.
