LETTER bombers struck again in North Yorkshire yesterday, posting a device to an agricultural business in Thirsk.

It's the tenth nail bomb sent through the mail since December - and police are convinced animal rights extremists are behind them.

A woman from East Yorkshire has still to discover if she has been blinded after a parcel detonated when she opened the post last month. A farmer from Ripon was lucky to escape uninjured the same day when another package exploded as he opened it at his kitchen table.

However, no one was hurt by yesterday's device, delivered to a firm which has not been identified for security reasons.

Staff were suspicious of the package and called police. They then called in Army bomb disposal experts from Catterick Garrison who carried out a controlled explosion.

But police were still urging others who could be potential targets to be on their guard. A spokesman said: "Anyone connected with agriculture or animals, either directly or indirectly, should treat their mail with caution.

"Any unsolicited packages which arouse suspicion should be left untouched and the police alerted.

"Members of the public with information which could lead to the capture of the bombers can also telephone the Crimestoppers line - (0800) 555111 - immediately and in confidence before anyone else is injured."

A firm in Masham in North Yorkshire, which makes equipment for vets, was the first to receive a letter bomb shortly before Christmas.

North Yorkshire's assistant chief constable is leading the hunt for those responsible, although the attacks are countrywide.

A fish and chip shop in North Wales, a pest control firm in Cheshire and a charity in Penrith have also been sent devices.