British organisations linked to motoring are being warned to be careful when opening their mail. Police are trying to find out who is responsible for the sending of seven letter bombs over the past three weeks. Anton Setchell from the Association of Chief Police Officers says it seems the intent was to shock but not kill.

?These devices do not contain conventional explosives, and although we are still awaiting the results of forensic analysis, the indications are that these devices are of a small pyrotechnic nature,? he said.

Six people were hurt on Wednesday by a mail bomb at the Driver and Vehicle Licensing Agency in Swansea. This was the third letter bomb in three days to hit companies or agencies with links to enforcing motoring laws. So far no group has claimed responsibility for the attacks. However, police say animal rights and motorist groups have been linked to some of the letter bombs.

<B>Angry motorist? </B>

Police believe three of the letter bombs ? one sent to a firm in Birmingham and two to firms in Oxfordshire, all on 18 January ? are linked. On the back of one of the envelopes was the name of Barry Horne, an animal rights extremist who died in 2001 while in jail for a firebombing campaign.

But letter bombs have also since been sent to motoring-related firms in London and Berkshire, and to a private address in Kent.

Officers are compiling a list of all the business activities and organisations associated with the companies targeted, to see if there is any link.

They are also examining the parcels and devices ? two of which were intercepted and are intact. Handwriting and postmarks are also being analysed.

The police?s national co-ordinator for domestic extremism said both animal rights activism and the possibility of a grudge-holding motorist were being examined as ?priority lines of inquiry?.

Assistant Chief Constable Anton Setchell added that the letter bombs did not contain conventional explosives, but were made up of pyrotechnic material designed to shock or cause only minor injury.

One of the four workers hurt in Wednesday morning?s incident had cuts to her hands and body. The woman said in a statement that she was ?shaken, shocked and frightened?.

On Tuesday a package exploded at the Berkshire offices of Vantis, an accountancy firm linked to a company which provides digital speed cameras to the police.

That followed a letter bomb attack the day before at the central London offices of Capita, which is responsible for the congestion charging system.

And on Saturday a letter bomb injured 53-year-old Mike Wingfield at his home in Folkestone, Kent.

It was addressed to the ?senior manager? of a now defunct security company he ran some years ago from his home address.

He said he had ?no idea whatsoever? why he was targeted.

Home Secretary John Reid described the incidents as ?worrying? and Prime Minister Tony Blair told the Commons that the attacks were being investigated ?very closely?.