A national police alert was issued yesterday after the seventh letter-bomb attack in three weeks.
Police suspect the attacks, six of which have been directed to businesses which supply services or technology to law enforcement agencies, are linked.
Both animal rights activists and the possibility of a grudge-holding motorist are being examined as "priority lines of inquiry", said the police national co-ordinator for domestic extremism, assistant chief constable Anton Setchell.
It is understood that at least two of the homemade pyrotechnic style bombs, all sent in A5 sized Jiffy bags, contained glass and one contained a piece of metal that could have been a nail. Mr Setchell, who issued a warning to office workers across the country, said all the packages had labels with a mixture of handwritten and typed addresses. Five have detonated when opened, injuring eight people. Detectives fear that someone could soon by maimed or killed.
The latest blast happened yesterday at the Driver and Vehicle Licensing Agency (DVLA) in Swansea. Four members of staff were taken to hospital, three of whom were discharged after being assessed for potential hearing problems.
One woman remained in Swansea hospital last night, in a stable condition after suffering injuries including cuts to her hands and body.
The woman, who asked for her name not to be disclosed, said: "I was shaken, shocked and frightened. Everybody started running around and I felt quite calm, but I didn't really know what had happened."
All incoming mail to the DVLA, the agency which maintains registers of drivers and vehicles and collects car tax, has been stopped in the wake of the blast.
This week has seen three letter-bomb attacks in consecutive days on companies connected to tackling motoring offences in London, Berkshire and South Wales.
A previous "cluster" of three bombs was sent just weeks before to forensic laboratories in Oxfordshire and the West Midlands area. One of the letters featured the name of a well-known animal rights extremist.
On Tuesday a letter bomb exploded at a firm linked to Speed Check Services, in Wokingham, Berkshire, a provider of digital speed cameras to the police.
That came after a letter- bomb attack the day before at the London offices of Capita, which runs London's congestion charge system.
While theories have been raised that the latest incidents are the work of a disgruntled motorist, the police now believe they are linked to four other attacks over the past three weeks, although they cannot say yet if just one group or individual is behind the campaign.
On Saturday, in an attack that appears to be separate from the two " clusters", a letter bomb exploded at a house in Folkestone, Kent, injuring Michael Wingfield, 53. He is believed to be a security company manager.
Mr Wingfield told the BBC: "There were three letters. I gave my wife two for my son and the other one... bang. There was a minor explosion and I felt the blast. Look down and you find out you are wearing a tea bag. There were bits of glass sticking out but apart from that, not a lot." He left his home in the Cheriton area covered in blood and glass after the device went off, said a neighbour.
Of the first three letter bombs, one was delivered to Orchid Cellmark in Abingdon and exploded. The firm deals in paternity testing. The other two to LGC Forensics in Culham, Oxfordshire, and the government-owned Forensic Science Service HQ in Birmingham were intercepted.
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