Two firefighters killed as a tower block blaze raged "completely out of control" were hailed for their bravery today.
James Shears, 35, and Alan Bannon, 38, died as they tried to contain the fire in Shirley Towers, Church Street, Southampton.
Police investigations were launched as fire chiefs said they could not yet confirm whether "something went wrong".
Hampshire Chief Fire Officer John Bonney said officers were "valiant" in "extremely difficult circumstances" but refused to give precise details of how they died.
Gordon Brown also paid tribute to their "bravery and commitment", sending his condolences to relatives.
Witnesses described how a ceiling and staircase fell through as the blaze spread. Dozens of people were evacuated and were still waiting to return to their homes tonight.
Mr Shears and Mr Bannon, who both served for more than seven years, were members of Red Watch at St Mary's fire station.
Mr Bonney, who visited the firefighters' families this afternoon, said colleagues were "devastated" on a "tragic day" for his brigade.
"I would like to pay tribute to the courage, the dedication and service of those firefighters," he said.
When asked about the circumstances of the fire, Mr Bonney said it was not appropriate to "speculate".
He added: "Whether something went wrong, we will discover in due course and if there is something that went wrong then we will address that... they were protecting the public under incredibly difficult circumstances."
The fire broke out in Flat 72 on the ninth floor at around 8pm last night. The flat was home to a single mother with two children, all of whom escaped uninjured, locals said.
Another two firefighters were taken to hospital by ambulance with minor burns to their hands.
Mother-of-two Kristina Baldacchino, who watched the incident unfold, said emergency services were struggling to get the blaze under control.
The 41-year-old, who runs the nearby Kings Arms pub, said: "I really can't understand what went wrong. We saw the fire engines arrive at what appeared to be a small fire in a flat. From the outside it really didn't seem that severe.
"Yet within an hour it was ripping through the entire block. It was a complete inferno and seemed to have gone completely out of control - even though Church Street was absolutely chock-a-block with fire engines, emergency services and even a crane.
"Some of the residents were told to stay in their flats, while others were evacuated.
"It was a complete tragedy and we had people into my pub afterwards - they were all very shaken."
The landlady said there had been a chip-pan fire in the block five to seven years ago which sparked similar scenes.
One of Mr Bannon's neighbours said: "He was a delightful dad. It's absolutely devastating news and we're so sad for his family."
Mr Bannon was married and had a little girl.
The Union flag was flying at half mast at St Mary's station while a steady stream of flowers was placed outside.
One card read: "You gave your lives keeping the public safe. The most precious sacrifice one gives to another. God bless."
Charlie Le Marechal, 22, was staying with his mother and brother, Ashley, on the sixth floor when the fire started. He told how he saw "flames coming out" of the floor above.
He added: "We saw glass falling from the flat on fire and down below we saw fire engines everywhere, ambulances and police."
Painter and decorator Ashley Le Marechal, 21, said he left the block when he was told the blaze had "got out of hand".
Nearby roads were cordoned off as 20 fire appliances and supporting vehicles from stations across Hampshire were sent to deal with the incident.
Those who were evacuated were provided with emergency accommodation at the local Salvation Army.
Both the Fire Brigades Union and the Chief Fire Officers' Association also paid tribute to the men, saying colleagues across the country were saddened.
A safety alert was sparked at the tower block in 2002 when two men plummeted 200ft to their deaths down a lift shaft.
The men, in their early 20s, were believed to have been involved in a fight after leaving a top-floor flat when they hit the lift doors - which sprang open.Reuse content