Firefighters and scientific police have begun inspecting the interior of two residential towers that were destroyed by fire in the eastern Spanish city of Valencia, killing four people, amid questions about how the blaze spread so rapidly.

Authorities said 14 people were still missing following the blaze at the complex on Thursday.

Valencia mayor Maria Jose Catala 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 United Kingdom and China.

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

Ms Catala said the cause of the fire was still not known and it was too early to comment on reports that suggested materials used in construction might have contributed.

Ms Puchades said polyurethane was no longer widely used in buildings without extra safety measures to offset its flammability.

The complex in Valencia was finished in 2009.

In a promotional video, the now bankrupt construction company Fbex boasted that it used a new aluminium-based material as part of its facade.

Fifteen people were treated for injuries and five remained in city hospitals on Friday, including four firefighters.

All were said to be stable.

Prime Minister Pedro Sanchez visited the scene, promising support for those affected and expressing gratitude to firefighters and military personnel who worked to extinguish the blaze.

It was not immediately known how many people were in the two buildings when the fire broke out, but dozens are believed to have lost their homes and belongings.

The complex had some 140 apartments.

The Valencia regional government declared three days of mourning and announced financial aid to cover accommodation, clothing and food.

Residents were housed overnight in hotels or in the homes of relatives and neighbours, authorities said.

Firefighters rushed to the scene on the outskirts of the city as flames burst from windows.

They used a crane to lift two residents from one of the balconies.

Some 90 soldiers from Spain’s Military Emergency Unit and 40 firefighting trucks were also deployed.

The fire sent clouds of black smoke billowing skyward that could be seen from miles away.

Spain’s weather agency, Aemet, reported winds of up to 60kph (40mph) at the time.