Police believe a fire which destroyed a partially-built timber frame building in Hampshire was caused by arson.
Four boys, three aged 15 and one aged 12, were arrested at the weekend in connection with the fire in Brighton Hill, Basingstoke.
Around 100 firefighters tackled the blaze, which broke out early on Friday evening at the four-storey block of retirement flats.
The fire damaged two cranes on the site, and left the structure on the verge of collapse.
Some 40 people were evacuated from nearby homes.
A series of fires over the past year have prompted criticism about the growing popularity of timber-frame, which is now the most common construction method in social housing.
John Bonney, Hampshire Fire and Rescue Service chief officer, said: "When timber-framed buildings catch fire the actual structure burns. It often leads to total collapse and that puts the safety of our firefighters at risk.
"The potential for fire spread was clearly of major concern at this incident, and shows that the Chief Fire Officers Association is right to be increasingly concerned about the number and severity of fires in timber-framed buildings under construction.''