Chevron Pembroke explosion: Carwyn Jones visits site
First Minister Carwyn Jones has visited the Chevron refinery in Pembrokeshire, a week after an explosion killed four workers.
He met emergency workers and staff and thanked them for their work in the aftermath of last Thursday's blast.
He also laid flowers at the site in memory of the four contractors who died in the blast.
Police say they are still working with experts at the scene to establish the cause.
"The events of last week have shocked us all," said Mr Jones.
Speaking during his private visit, to which media were not invited, Mr jones sent his condolences to the bereaved families and their colleagues.
"I have come here to personally thank the emergency services and Chevron staff who worked tirelessly following this tragic incident," said Mr Jones.
"This plant plays a major part in the community, providing jobs for families across the region," said Mr Jones.
"We must now let the official investigation take its course and learn any lessons from its outcome."
Julie Jones, 54, of Pembroke, and Dennis Riley, 52, Robert Broome, 48, and Andrew Jenkins, 33, all of Milford Haven, were killed in the explosion.
Refinery owners Chevron said the incident occurred while a storage tank was being taken out of service for maintenance.
Dyfed-Powys Police said another worker badly injured in the blast is in a stable condition at Morriston Hospital in Swansea.
Chevron, which has flown in experts from the US, said the investigation would be a "lengthy process".
Speaking during Prime Minister's Questions on Wednesday, David Cameron described the explosion as a "tragic incident".
Meanwhile police have said they are continuing to work with the Health and Safety Executive and forensic experts at the scene.
A spokesman said family liaison officers were continuing to support the families of the deceased.
The inquest into the deaths is expected to be opened on Monday at Milford Haven Town Hall.