Six people were the latest victims yesterday, suffering injuries in an explosion at the Driver and Vehicle Licensing Agency (DVLA) in Swansea – the seventh homemade device posted in three weeks.
Detectives, who believe the devices are connected, said they were pursuing "priority lines of inquiry". These include links to animal rights extremists and the possibility an angry motorist was waging a campaign against traffic enforcement organisations.
Staff at DVLA offices across the country including Leeds, Sheffield and Beverley have been ordered not to open post until further notice as businesses, Government agencies and the public were urged to take extra precautions.
At least nine people have been injured in total in the attacks. It is understood that at least two of the devices contained glass. One contained a piece of metal that could have been a nail.
Yesterday's blast in the post room at the DVLA's main centre left a woman with burns as well as cuts to her hands and body, although her injuries are not life-threatening. Three others were treated in hospital for hearing problems and two more on site.
Colleagues said they had heard it was a small firework device in a padded bag.
One said: "We are absolutely petrified here. Everyone is really frightened."
Mark Serwotka, general secretary of the Public and Commercial Services Union, said: "It is completely unacceptable that people who serve the public have been targeted in such a way and we urge staff across the whole of the civil service to be extra vigilant.
"We are calling for a risk assessment of post opening in all DVLA offices, as well as an urgent review of post room security across the whole of the civil service."
At Prime Minister's Questions yesterday, Tony Blair expressed sympathy to all those caught up in the "traumatic" attacks which, he said, were being investigated "very closely".
Local Government Association chief executive Paul Coen said: "We can only hope that the perpetrator of these bombings is caught by the police before someone is seriously injured or killed."
On January 18, three packages were received by two forensic science companies in Oxfordshire and another in Birmingham. One featured the name of a well-known animal rights extremist.
On Saturday, a letter bomb exploded at the house in Folkestone, Kent, injuring the 53-year-old security company director who opened it.
Another device exploded on Monday at the London offices of Capita, which runs the capital's congestion charge system, and the next day another went off at an accountancy firm linked to a provider of traffic monitoring devices in Berkshire.
Assistant Chief Constable Anton Setchell, the national co-ordinator for domestic extremism, said the devices in padded A5 envelopes were of a "small, pyrotechnic nature" designed to shock rather than kill and were directed at businesses rather than individuals.
"We are at an early stage of our investigation and I want to keep a completely open mind about the motivation," he added.
He said there had been no threats, no ransom demands and no claim of responsibility.
Companies, organisations and individuals should take "extra care" when handling mail and use postal screening measures, he added.
An anti-speed camera campaigner known for attacking roadside cameras last night denied any involvement.
The anonymous vigilante known as Captain Gatso said: "It's not me or my group. I condemn it completely. We only damage speed cameras, we don't damage people."
mike.waites@ypn.co.uk