As firefighters and scientific police Friday began inspecting the interior of two residential towers that were destroyed by fire in the eastern Spanish city of Valencia, killing four people, questions abounded as to how the fire spread so rapidly.

Fourteen people were still missing following the blaze at the complex on Thursday.

Valencia mayor María José Catalá said that the danger of the 14-storey building collapsing and the intense heat from the fire initially prevented emergency workers from getting in to search for possible survivors. The fire started on Thursday evening and quickly engulfed the buildings. Experts said that the building’s polyurethane cladding may have contributed to the fire’s ferocity.

The vice-president of the Valencia College of Industrial and Technical Engineers, Esther Puchades, who once inspected the building, told the state news agency Efe that when the material “is heated it is like plastic and it ignites”.

She said it was the first fire of its type in Spain, but that other blazes involving the material have been similarly destructive in the UK and China.

The 2017 fire at Grenfell Tower in London, which also had polyurethane cladding, caused more than 70 deaths.