DUBAI, United Arab Emirates, (AP) – A massive explosion at a fireworks factory left two people dead and two injured Wednesday, while subsequent fires rapidly spread, sending thick clouds of black smoke into the sky, officials and witnesses said.

It was unclear what caused the blast in the Al Qouz Industrial area of this booming Gulf city-state, said a civil defense official who requested anonymity because he was not authorized to speak to the media.

Witnesses said there was an initial large blast followed by several smaller ones in the area, which consists of warehouses, factories, malls and labor camps inhabited by thousands of Asian construction workers. Firefighters battled raging flames, which spread to other warehouses due to persistent winds, destroying four of the structures.

“Fire spread really quickly and the sun disappeared behind terrible smoke,” said Sunber Raj, a laborer from India who was walking to the bus stop to go to work when the explosion occurred.

Authorities said one person was immediately killed in the explosion and officials at nearby Rashid hospital said a second person died later at the hospital of his injuries. One of the injured was being treated at the hospital.

Ambulances and fire engines struggled to the scene through the morning rush hour, while helicopters hovered above the ruined factory.