Fears that an enraged motorist has begun a concerted letter bomb campaign were apparently confirmed today after a third attack on motoring-related companies in three days.
Home Secretary John Reid described the series of attacks, in consecutive days, as "worrying".
An investigation has been launched by the national co-ordinator for domestic extremism (NCDE) Assistant Chief Constable Anton Setchell.
The latest blast happened just after 9am today when a package exploded at the Driver and Vehicle Licensing Agency’s main centre in Swansea.
A woman in the postroom was injured in the blast and taken to hospital.
Yesterday, a package exploded at the offices of an accountancy firm linked to Speed Check Services, a provider of digital speed cameras to the police.
That followed a letter bomb attack the day before at the central London offices of Capita, which runs London’s congestion charge system.
Following the latest explosion at the DVLA, Mr Reid said: "Naturally, these incidents are worrying.
"It is important that we allow police to get on with their investigation without undue speculation."
Mr Setchell is attached to the Association of Chief Police Officers (Acpo) and will co-ordinate the police investigation into the bombings.
Three police forces are now involved in the nationwide hunt for the bomber.
A South Wales Police spokeswoman said a woman was injured in the latest blast, which she described as "a small letter-type explosion", at around 9.23am today and taken to hospital.
Her injuries are not thought to be life threatening.
"Police are in attendance following an incident at the DVLA offices in Swansea, where a small explosion has been reported," she said.
"One female has been taken to hospital with injuries, which are not believed to be life-threatening. A cordon has been put in place and nearby residents are being evacuated as a precautionary measure.
"Emergency services remain at the scene as further investigation is carried out."
A DVLA spokeswoman said the building was not evacuated as a result of the explosion.
The DVLA is an executive agency of the Department for Transport.
The Agency’s primary aim is general motoring law enforcement - maintaining registers of drivers and vehicles, and collecting car tax.
Welsh Conservative AM Alun Cairns said he was shocked and horrified at news of the letter bomb in Swansea.
Mr Cairns, who is the Assembly Member for South Wales West, said: "All of my sympathy and best wishes go out to the woman involved and her family.
"I have tabled an urgent question to the Presiding Officer asking for a statement on the incident and the implications it has for the safety of staff at the DVLA as well as members of the public.
"We will be setting up meetings with the DVLA, police and staff in order to assess the consequences of this attack and ways of protecting personnel in the workplace."
The first letter bomb exploded at the offices of business outsourcing company Capita in Victoria Street, central London.
The bomb, stuffed into a padded bag, exploded in the hands of a worker in the post room, leaving her with blast wounds to her hands and stomach.
She was taken to hospital and is said to be making a good recovery.
Scotland Yard’s counter terrorism unit is investigating Monday’s blast.
Then yesterday, a letter bomb exploded at the offices of accountancy firm Vantis on an industrial estate in Wokingham, Berkshire.
The package was not addressed to the company.
It later emerged that one of its clients is Speed Check Services, based in Camberley, Surrey, which is the sole provider of digital speed enforcement technology to police and highways authorities.
That explosion left two men with blast injuries to their hands and upper bodies, but they did not need hospital treatment.