Specialist teams have resumed their "inch by inch" search for three missing firefighters following a suspicious warehouse blaze in Warwickshire.
One firefighter has already been confirmed dead following the fire in Atherstone-on-Stour on Friday evening.
The county fire service chaplain named the men as Darren Yates-Badley, Ashley Stephens, Ian Reid and John Averis.
Warwickshire Police said on Monday it had not yet established the cause of the fire which "remains suspicious".
Canon David Capron praised the men's "loyalty and heroism" at a service in Alcester on Sunday.
Mr Capron, who is from a family of retained firefighters, said Mr Reid and Mr Averis were based at Stratford-on-Avon fire station, while Mr Yates-Bradley and Mr Stephens were based at Alcester fire station.
Chief Superintendent Paul Mason Brown of Warwickshire Police told a news conference on Monday that the cause of the fire was still being investigated and he appealed for any information from the public.
Assistant Chief Officer Jon Hall, of Hereford and Worcester Fire Service, told the conference their search had so far covered around half of the first floor area.
"We're currently trying to clear the entire ground floor area and our search continues," he said.
"To be honest this is absolutely uncharted territory. There was a significant movement of a structural member inside the building this morning that did force us to constantly revaluate our presence in the building."
One of the four was confirmed dead after being pulled from the fire. Three of his colleagues remained in the burning premises as the roof collapsed.
The search operation, involving more than 100 fire officers from Warwickshire, Hereford and Worcester and the West Midlands, began on Sunday.
Avish Malde, a director of site-owner Wealmoor Atherstone Ltd, has revealed there was no sprinkler system installed at the vegetable-packing warehouse.
None of the 300 people the firm employs at the warehouse are missing.
Mr Malde, who said the fire began at the end of a shift, said: "There isn't a fire sprinkler system in place at the moment but plans were in place to actually put one in.
"So at this moment in time we're co-operating fully with the fire services on that line of the inquiry."
The Mayor of Alcester, David Hancox, said: "It's a very difficult time for the town. I think the shock that we experienced on Saturday morning is changing now to one of grief and the town seems to be under a cloud I think at the moment.
"We shall pull rank on the families and give them all the support that we possibly can."
"It's one of those communities where everybody knows everybody. I would describe it more as a family than strictly a community."
Specialist teams are investigating why the fire crews went into the warehouse while it was still burning.
The Fire Brigades Union (FBU) said it believed all four men were retained firefighters, who work part-time.
Steve McGuirk, president of the Chief Fire Officers' Association, said: "This dreadful incident is a sharp and poignant reminder to us all of the risks faced by our firefighters on a daily basis and that there is no such thing as a routine incident."
The incident in Warwickshire could be the worst loss of life for on-duty firefighters for more than 30 years.
Seven firefighters died in 1972 while fighting a blaze at a warehouse in Kilbirnie Street, Glasgow.