DAMIEN HENDERSON damien.henderson@theherald.co.uk THE full extent of a letter-bombing campaign became clear last night as police revealed details of seven linked attacks in the last three weeks.

In the latest incident, four workers from the Driving and Vehicle Licensing Authority (DVLA) were taken to hospital yesterday after a package exploded in the postroom of its headquarters in Swansea at around 9.30am.

Homemade bombs in A5 padded bags have been posted to seven different businesses in England and Wales since January 18 - six of which supply services or technology to law-enforcement agencies. Nine people have been injured.

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".

Though no attacks have taken place north of the border, the Association of Chief Police Officers in Scotland (Acpos) said office workers should be vigilant when handling mail and report any suspicious packages to the police immediately.

The DVLA said yesterday staff in its postroom were warned similar devices had been sent to other motoring organisations. Three mail bombs were sent to forensic laboratories in Oxfordshire and the West Midlands last month and another three to traffic-enforcement companies in London, Berkshire and south Wales this week.

The other was sent to a house used as a business address in Folkestone, Kent, on Saturday.

It is understood at least two of the devices contained glass. One contained a piece of metal, possibly a nail.

Detectives believe there is a link between all devices.

There have been no threats, no ransom demands and no claim of responsibility. Each device was apparently targeted at the business, as none had featured the name of a specific individual.

Assistant Chief Constable Anton Setchell said the intention of the bombs was clearly to cause shock and fear. At a press conference last night, he said: "The investigation into each package that has been received has been linked from the outset.

"This is now a very substantial investigation involving a number of forces, including regional and national specialist police resources."

Companies, organisations and individuals should take "extra care" when handling mail and use postal screening measures, he said.

The bombings began last month and there have been two "clusters" and one apparently separate incident.

On January 18, three packages were received by forensic science companies in the Thames Valley and West Midlands area. On Saturday, a letter bomb exploded at a house in Folkestone, the business address of a man believed to be a security company manager responsible for providing door staff and security services to businesses.

On consecutive days this week, letter bombs have exploded at motoring-related companies, including the one at the DVLA's main centre in Swansea. The agency maintains registers of drivers and vehicles, and collects car tax.

Mr Setchell said all the packages had labels with a mixture of hand-written and typed addresses. They were designed to activate when the contents are pulled out and were "pyrotechnic" in nature, he said.