Ashley Greene Condo Fire -- Candle Triggered Blaze

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.