A candle started the fire at Ashley Greene's condo

We now know what caused the fire at Ashley Greene's condo that killed her beloved dog -- an unattended candle ... TMZ has learned.

Law enforcement sources tell us ... they have already done a preliminary investigation and determined the candle lit the living room couch on fire -- a fire which spread throughout the unit.

As we previously reported, Greene, her boyfriend and brother were all sleeping in the condo when the blaze erupted.

In addition to the dog ... 3 people in the building were injured.