A member of staff at the DVLA in Swansea has been taken to hospital after a parcel bomb exploded.
The attack happened in the post room at about 0920 GMT and is the third of its kind on driving-related companies in as many days.
The bomb could confirm fears that a motorist is mounting a campaign.
Home Secretary John Reid said the incidents were "worrying".
A total of four people have been injured in similar attacks this week.
On Tuesday, two men suffered blast wounds in the explosion at the offices of Vantis in Wokingham.
And on Monday, a worker was hurt by a device at the offices of Capita, which runs London's congestion charge system.
The DVLA maintains UK-wide registers of drivers and vehicles.
The Welsh Ambulance Service said they were called at 0921GMT and arrived five minutes later.
A spokeswoman said one woman had been taken to Morriston Hospital. Her injuries are not known, but they are not thought to be life-threatening.
Cordoned off
Morriston NHS Trust said: "A Swansea woman was brought to the A&E department at Morriston hospital shortly after 10am following an incident at the DVLA.
"She is undergoing treatment at the present time and has been assessed by a consultant. Her condition is stable.
"Other casualties are being assessed at the scene by ambulance staff and A&E at Morriston is on standby is case they need to come to hospital."
Police are stopping people from entering the building and the area has been cordoned off.
A DVLA spokesman said: "A female member of staff was injured and the lady is on her way to hospital as a result of the incident."
About 4,000 people work in the DVLA offices in Swansea, but the building has not been evacuated.
The incident in Swansea follows an explosion on Tuesday at the offices of an accountancy firm linked to Speed Check Services, the provider of digital speed cameras to the police.
That followed a letter bomb attack the day before at the central London offices of Capita, during which a female worker received minor injuries.
Home Secretary John Reid said: "Naturally, these incidents are worrying. It is important we allow police to get on with their investigation without undue speculation."
Assistant Chief Constable Anton Setchell, the national coordinator for domestic extremism, is leading the investigation into the letter bombing campaign.