The Fire Brigades Union has said it will conduct its own investigation into the deaths of two firefighters killed in a tower block blaze.
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 on Wednesday as fire chiefs said they could not yet confirm if "something went wrong".
The Fire Brigades Union said 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, from Poole in Dorset, 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 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.
