Emergency services have found the bodies of two men after an explosion at a steelworks in south Wales, police said.
Five other people were injured in the blast at the two of whom remain in hospital.
Superintendent Stephen Jones, of South Wales Police, told reporters at the scene: "Two bodies have been found. A recovery operation is now under way and police, fire and ambulance personnel remain at the scene."
Mr Jones said that of the five injured in the explosion at around 10.30am, one person was treated and discharged at the scene and four were taken to the University Hospital of Wales, Cardiff.
"Two of these patients have been discharged and the other two remain at A&E," he said.
"The families of two men have been informed and they continue to be supported by family liaison officers. The Health and Safety Executive inspector is currently at the scene supporting our initial inquiries. Our focus is on the recovery of the bodies, supporting the families and finding out what caused this to happen."
Secretary of State for Wales, Stephen Crabb, said: "This has been a truly devastating day for the families of the two Celsa employees who tragically lost their lives. I want to send my sincere condolences to them and to their colleagues at this extremely difficult time. Our thoughts are also with those who were injured.
"The plant is a key feature of the south Wales industrial landscape and today's events have touched us all.
"As the families grieve for their loved ones, and those injured in the blast begin their recovery, we must now let the investigation run its course and ensure that lessons are learned for the future."
Eyewitnesses said a loud bang shook nearby buildings before thick black smoke billowed into the air.
A spokeswoman for Celsa said: "Our thoughts and deepest sympathies are with those affected and their families, who have been informed. Our utmost priority is to support all of those involved, and we are in close contact with the University Hospital of Wales, where the injured are being cared for."
The number of ambulances and fire engines reached more than two dozen as well as an air ambulance being deployed.
One man working at a nearby site said: "There was a massive bang. We all felt the floor shake. It was very worrying seeing lots of ambulances and fire engines go in there."
Steelworkers' union Community said it believed the blast came from the bar and rod mill at the site and may have involved an oil accumulator in the basement of the building.
A spokesman added: "Our immediate thoughts are with the individuals affected and their families.
"A member of our regional team is on his way to the site now to see what assistance we can offer and to seek further clarity on what has happened."
Ian Greenman, area manager for South Wales Fire and Rescue Service, said it was "an extremely difficult and arduous incident involving an explosion in the basement".
He added: "Crews worked tirelessly to effect rescues and deal with the fire. Despite heroic attempts, unfortunately we were unable to save two on-site personnel. All the thoughts of South Wales Fire and Rescue Service are with the families of the workers involved in the incident today."
Chris Sims, head of operations, resilience, for Wales Ambulance Service, said: "Our thoughts are with everyone affected by today's incident at Celsa Steelworks. We had a thorough and robust response to this incident. Among resources dispatched to the scene were the Trust's hazardous area response team, as well as colleagues from the Welsh Emergency Medical Retrieval and Transfer Service. We worked with our police and fire colleagues to help locate casualties and provide treatment before taking them to hospital."
Health officials said while its "major incident protocol" for the incident had ended, it warned there still could be delays for non-urgent patients attending the University Hospital of Wales.