A firefighter has died and two others have been injured after being engulfed in a fireball in an explosion at a burning building in Cardiff.
Richard Jenkins' colleagues tried to save him, despite their own injuries
Arson has not yet been ruled out, and police said they were treating the investigation as seriously as a murder inquiry.
The fire has been described as "suspicious", and a joint investigation is being carried out by the police, fire service, and the Health and Safety Executive.
A compressed gas cylinder found inside the building may have caused the explosion, but the exact cause of the fire remains unconfirmed.
Richard John Jenkins, 29, was married with two young children and lived in Bridgend. He had been a fire-fighter for nine years and had recently moved to the Barry station, where he was a member of the White Watch team.
He leaves a wife, Kathy, and a son aged nine and daughter aged six.
Relatives and friends have visited the scene of the tragedy and have left floral tributes.
Emergency services were called to a fire at a disused bingo hall and community centre on Bishopston Road in the Ely area of the city at 0110 BST.
Crews were sent from Ely and Barry to tackle the blaze, and were on the scene within minutes.
Mr Jenkins went in with a hose and, within seconds, there was an explosion and a fireball engulfed him.
His two colleagues were also caught up in the flames and were badly burned. Despite their own injuries, they still went inside to rescue their colleague.
The two men fought to resuscitate him, but he was found to be dead on arrival at hospital.
The other two officers - one who lives in Ely and one from Barry - were released from hospital after treatment.
Investigations are continuing, but fire officers are exploring views that the fire may have been started deliberately.
"The fire is being treated as suspicious by the police and we are working closely with teams of crime officers, forensics and the Health
and Safety Executive so we can fully understand what has happened," said Andy Marles, Deputy Chief Fire Officer for South Wales Fire and Rescue Service.
Colleagues of Mr Jenkins have been left shattered by the tragedy.
"All of us are absolutely devastated," said Divisional Officer Mick Flanagan, who is head of fire investigation.
Fire investigators are examining the building
"I deal with a lot of fatal fires, and they are always sad, but when you attend a fire and the fatality is one of your own, it's unbelievably painful," he explained.
The area around the burned-out building - which had been empty for about a year - has been sealed off so that the structure can be made safe and scene of the fire can be examined.
Detective Chief Inspector Brent Parry said 30
detectives were interviewing in the area.
"We are treating it on the same level
as a murder inquiry," he said.
"We are trying to ascertain any events which led up to the incident. Officers are working on house-to-house inquiries and speaking to witnesses
in the area."
Residents living nearby have told police that gangs of youths often gathered around the site.
Haydn Harries, who lives nearby said he was woken by the blaze.
"I could hear this crackling and banging as if someone was letting off fireworks in the street," he said.
"I came out to have a look and the old community hall, which is all wood, was on fire.
"It was a shock to hear there was a fireman caught in that explosion."