The Fire Brigades Union said it will conduct its own investigation into the deaths of two firefighters, including one from Dorset, killed in a tower block blaze.
James Shears, 35, from Poole, and Alan Bannon, 38, died as they tried to contain the fire in Shirley Towers, Church Street in Southampton on Tuesday night. Police investigations were launched on Wednesday as fire chiefs said they could not yet confirm if "something went wrong".
The Fire Brigades Union said on Wednesday night it is carrying out its own inquiry into the causes of the deaths, and would share its findings with the police and the Health and Safety Executive.
General secretary Matt Wrack said: "We want to find out what happened urgently, so that we can take steps to ensure that it never happens again. I have already spoken to the police to assure them of our support and co-operation."
Hampshire chief fire officer John Bonney said officers were "valiant" in "extremely difficult circumstances" but refused to give precise details of how they died. Mr Shears left a widow Carla and two young boys and Mr Bannon, from Southampton, was married to Charlotte and the couple had a young daughter. The grieving loved ones of both men visited the scene of their deaths on Wednesday as they tried to cope with the loss.
Witnesses described how a ceiling and staircase fell through as the blaze spread last night in the 1960s building. Dozens of people were evacuated and were still waiting to return to their homes, with some set to be put up in hotels.
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 yesterday 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 said: "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 on Tuesday. 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.
Both the Fire Brigades Union and the Chief Fire Officers' Association paid tribute to the men, saying colleagues across the country were saddened. Gordon Brown paid tribute to their "bravery and commitment", sending his condolences to relatives.