Yesterday officials announced an arrest into the massive fire of the Da Vinci apartment complex that raged near downtown Los Angeles last year. 

The incident caused $20 million to $30 million in damage to the apartment complex and more than $50 million in damage to a nearby city-owned building. 

But how did authorities know it was arson? And once arson is suspected, how do they look for clues that would lead them to a suspect?

We talked to Laura Billon, she's a certified fire investigator and arson profiler for the Center for Arson Research.