An operation is taking place to recover three firefighters missing presumed dead following a warehouse blaze.
One firefighter was confirmed dead after being pulled from the building in Atherstone on Stour, Warwickshire, at the height of the blaze on Friday.
His three colleagues remained in the burning building as the roof collapsed.
Experts will try to discover the cause of the fire, which is a suspected arson attack. Fire chiefs will also look at why the crews went into the warehouse.
Officers from the fire service briefly entered the building on Saturday to search for the three missing men.
They spent a few minutes inside to conduct an "initial visual assessment".
The Fire Brigades Union (FBU) said all four were retained firefighters, who work part-time.
The operation on Sunday will involve specialist fire officers with the assistance of structural engineers, to make the building safe, and the Health and Safety Executive.
Sir Ken Knight, the government's chief fire and rescue adviser, said the incident was "harrowing".
He said the investigation would focus on the cause of the blaze and the fire prevention measures at the site.
Warehouse workers managed to escape the blaze when fire broke out, police have said, but it is not known whether fire crews believed there may be more civilians inside the building when they entered.
The fire service's chaplain Canon David Capron, who is from a family of retained firefighters, said people were converging on nearby Alcester's fire station in tears to express their condolences.
He said: "Clearly this has deeply affected the town. It's a close-knit community."
A service was held at St Nicholas's Church in Alcester on Sunday morning for the firefighter who had died and those missing.
People were seen silently walking into the church holding hands and with bowed heads.
A Warwickshire fire service statement said: "Concerns are heightened hour by hour for the three firefighters who are still missing.
"Our worst fears may be realised but we will not give up our search."
The gutted warehouse is being treated as a crime scene and police said they believed the fire might have been started deliberately.
Prime Minister Gordon Brown said he was "shocked and saddened" while Conservative leader David Cameron said it was a reminder of the dangers facing emergency workers.
Warwickshire's chief fire officer William Brown, speaking at a press conference on Saturday, described it as "the worst day of my career".
The crew may have entered the building because people were believed to be inside, he said, but also because the initial fire when they arrived could have been much smaller than what it became.
Vegetable packing and distribution firm Wealmoor Atherstone Ltd owns the site, where it employs 300 people.
A spokesman for the company said it was "devastated" by the death of the firefighter and would cooperate with police and fire investigations into the blaze.
Matt Wrack, general secretary of the FBU, told BBC News the potential loss of four lives would make the incident the worst loss of life among its members for 35 years.
In 1972, seven firefighters died while fighting a blaze at a warehouse in Kilbirnie Street, Glasgow.
In that incident, one firefighter was trapped inside the building and six of his colleagues died trying to rescue him.