The Fire Brigades Union has said it is starting its own inquiry into the deaths of two firefighters killed tackling a blaze at a block of flats.
James Shears, 35, and Alan Bannon, 38, died at the 15-storey Shirley Towers in Church Street, Southampton.
FBU 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."
Fire personnel are due to lay wreaths at Shirley Towers later.
Hampshire Chief Fire Officer John Bonney, who described Mr Shears and Mr Bannon as "valiant" in "extremely difficult circumstances", is expected to be among those who will attend the event.
Both the deceased were full-time firefighters and members of Red Watch at St Mary's fire station.
Mr Shears, from Poole in Dorset, had been with the service just under seven years; Mr Bannon, from Southampton, for eight-and-a-half years.
The precise details of how they died have not yet been released.
The FBU has said it would share its findings with police.
It also plans to show the report to the Health and Safety Executive.
Mr Wrack said: "I have already spoken to the police to assure them of our support and co-operation."
Two other firefighters suffered burns to their hands in the blaze which started on Tuesday. All the residents of the flats were rescued.
Eyewitnesses describe the scene - Mobile footage captured the flames
Eyewitnesses described how a ceiling and staircase fell through as the fire, which started on the ninth floor, spread through the building.
Everyone was eventually evacuated from the high-rise, one of Southampton's tallest, and dozens of people were taken to a nearby emergency rest centre.
The flat where the fire began was home to a mother with two children, who were all unhurt, residents said.
Mr Bonney, who visited the firefighters' families on Wednesday, told a news conference: "We are devastated by the loss of our colleagues who died in the line of duty.
"Our thoughts are with their families, with their friends and their colleagues who have all been stunned by these events.
"I would like to pay tribute to the courage, dedication and service of those firefighters."
He added: "At this stage it is not appropriate to speculate around circumstances relating to this incident.
"Whether something went wrong, we will discover in due course, and if there is something that went wrong then we will address that."
The union jack flag has been flying at half mast on fire stations across the country, and floral tributes have been laid at St Mary's fire station.