A fire that reduced an unfinished downtown apartment building to a block-long mountain of blackened wood was an act of arson that caused an estimated $20 million to $30 million in damage, the Fire Department announced Thursday. The Dec. 8 fire burned the seven-story, 1.3-million-square-foot Da Vinci complex. Investigators from the Fire Department and a national response team from the Bureau of Alcohol, Tobacco, Firearms and Explosives sifted through 75,000 square feet of debris to find the cause, a Fire Department statement said.