UPDATE: Box of candles started fire NY Daily News reports.
The Serbian Orthodox Cathedral of St. Sava, in the Flatiron district, was home to a congregation of more than a thousand, and served as a gathering place for New Yorkers of Serbian descent for decades. A few hours after gathering for the Orthodox Easter service, the building was engulfed in flames, in a 4-alarm fire that required more than 170 firefighters and left the building in ruins.
The national historic landmark site was originally the Trinity Chapel, home to an Episcopal congregation, before it was sold in 1942. It had been built from 1850-55 and was designed by Richard Upjohn, the famous architect. It survived a nearby bomb blast in 1973, after which the shattered stained-glass windows were replaced with Byzantine windows, and had undergone interior renovations to resemble Byzantine style.
Many news outlets have written about the fire, which is being investigated, and only resulted in one minor injury.
New York Times (video, article)