HANGZHOU - Sunday's factory explosion in Wenzhou city, Zhejiang province, that left 16 injured, was caused by sparks from aluminite powder, local authorities said Monday.

By Monday morning, seven of the 16 remained in critical condition. Most having suffered large-area burns.

The Wenzhou production safety department said the explosion was caused by fire that broke from aluminite powder stored at the factory, which is a raw material for making polishing products.

The powerful explosion happened at 1:28 pm in the city's Lucheng district, razing the upper floor of a two-story polishing factory and leaving the site in heavy smoke, said one rescuer.

The fire was contained in two hours, and firefighters evacuated more than 100 residents.

The accident is still under investigation.