Police have now confirmed they believe six letter bomb attacks, including two in North Yorkshire, are the work of animal rights extremists.
Senior investigators are linking all six incidents, two of which took place in North Yorkshire and the others in East Yorkshire, Cheshire, North Wales and the West Midlands.
Following a conference involving senior officers conducting the investigation, police confirmed that their inquiries were now concentrated on animal rights activities.
Police also said they had called in the assistance of the Metropolitan Police anti-terrorist squad.
Peter Walker, deputy chief constable of North Yorkshire Police, is overseeing the investigation.
He said: "These have been indiscriminate criminal attacks on people going about their lawful business.
"While those responsible may wish to claim they are acting for some higher purpose, every right-minded person will express outrage at what has been done."
Updated: 08:34 Thursday, January 18, 2001
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