MADRID — At least 17 people were injured when a four-story building exploded in central Madrid.

Mayor José Luis Martínez Almeida said four people were taken to the hospital and rescue teams were continuing to search for people who could be trapped in the building.

Firefighters said there is a possibility the building could collapse.

It’s still unknown what caused the explosion, but TeleMadrid reports that witnesses smelled a strong gas odor.

Firefighters believe the explosion may have happened in the attic or in one of the upper floors of the building.