Young people were seen running from a farm store on the outskirts of Dundee shortly before the building was engulfed in flames, sending plumes of smoke high into the sky.
One of the group stopped to use her mobile phone to alert Tayside Fire and Rescue before running off after the group suspected of starting the blaze.
A 10-year-old girl who arrived shortly after with her mother told how the corrugated iron roof of the building, in a field adjacent to the Clearwater Park housing development off the A92 Dundee-Arbroath road, “just melted” and collapsed as firefighters were training their hoses on it.
Watch manager Paul Perrie from Balmossie Fire Station, who led the team tackling the blaze which erupted shortly before 8pm on Wednesday, said it was too dangerous to send the firefighters in.
He said, “The crews were not committed to the building, due to the unsafe nature of it.
“You would not be able to save that building and it’s not worth risking a life on it.”
With flames shooting out of the building and a massive plume of thick black smoke billowing an alert that could be seen from some distance, a crowd of onlookers soon gathered on Linlathen Road.
One man said he plucked his two small sons from their beds in Strathyre Avenue on Clearwater Park and took them across to see what was going on.
Five-year-old Greg Ramsay and his brother Ross (4) stood in their pyjamas while the flames licked through the roof of the store.Roof “melted”Carole Sommerville said she saw the smoke from her home in Largo Place, around three quarters of a mile away.
She was concerned that a friend’s home in Clearwater Park had gone up in flames and rushed over with her 10-year-old daughter Neve, to discover it was a large Nissen hut in the field on the opposite side of Linlathen Road from the Clearwater Park estate that was on fire.
Neve said, “The firefighter came with an axe and whacked the door down and there were flames coming out of the top.
“The middle bit of the roof just collapsed, it just melted.”
The firefighters from Balmossie and Kingsway Fire Station pumped water from the nearby Dighty Burn.
A woman, who wished to remain anonymous, said she was out walking her dog when she saw young people running from the Nissen hut which is believed to have been stacked with straw.
She said, “I heard crashing and banging and then saw flames coming through the walls and a group came running up.
“They were in a state.”
The dog-walker said she accused them of “wilful fire-raising.”
A girl who had been with the group at the hut said she was not involved and gave the names of two males she claimed started the fire.
However, the dog-walker was more interested in alerting Tayside Fire and Rescue, adding, “I didn’t have a mobile so I told her to use hers and dial 999 and that is what she did.”
A Tayside Police spokesman said there would be a joint investigation with the fire service into the incident.