Four people injured after a parcel bomb exploded at the Driver and Vehicle Licensing Authority building in Swansea have been released from hospital.
The explosion happened in the post room of the city offices shortly after 0920 GMT - one of seven devices received across the UK in the last three weeks.
At the DVLA, one woman suffered minor burns and two men and a woman were treated for hearing-related injuries.
One of the victims said she had been "shaken, shocked and frightened".
The attack at the DVLA, which maintains UK-wide registers of drivers and vehicles, follows others on motoring-related companies this week in Wokingham and London.
The Welsh Ambulance Service said they were called at 0921 GMT and arrived five minutes later.
People were stopped from entering the building, where 4,000 people work, and the area was cordoned off.
An army bomb disposal squad escorted by two police cars arrived at 1300 GMT.
One of the victims, who received hospital treatment for injuries including cuts to her hands and body said in a statement: "I was shaken, shocked and frightened. It's hard to describe how I felt.
"Everybody started running around me and I felt quite calm, but I didn't really know what had happened."
South Wales Police called for vigilance over the possibility of further attacks.
Divisional commander, Mark Mathias, said: "There is no intelligence to suggest that anyone else is currently at risk and we would appeal for calm.
"However workers in relevant posts are urged to be vigilant, and to carry out their work with increased caution. "If you have any suspicion about any package, ensure you leave it unopened and unhandled, and contact your local police station or Crimestoppers."
He added: "Our priority today has been the safety of those working in the DVLA offices and also the community as a whole.
"Throughout this incident, the security measures have been continuously assessed and the decision was taken to evacuate all staff from the mail room.
"This room is isolated from the main offices, so there was no need for any further evacuation."
Swansea NHS Trust said all four of the people injured were from the Swansea area.
Police cordoned off the area around the DVLA in Swansea
Another member of staff at the site, Vivien Hodge, said workers had been told to report as normal on Thursday.
She added: "They tried to reassure us that the building is safe, yet they are not letting evening shift come in tonight.
"They are bringing sniffer dogs in and there are a lot of nervous people working there."
Conservative AM for South Wales West, Alun Cairns, said he was horrified by the attack.
He said: "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."
Swansea councillor Ray Wellsby said there had been concerns regarding security in the building in the past.
"Obviously with the redevelopment that's going on inside the building... more people will be coming into the area to do business with the DVLA and therefore we want greater security."
Assistant Chief Constable Anton Setchell, the national coordinator for domestic extremism, is leading the investigation into the apparent letter bombing campaign.