Officials have ruled that a March fire on a Manhattan movie set that killed a city firefighter started due to heat from a boiler, and that the building's sprinkler system had been shut off.

Fire marshals said Tuesday afternoon that heat from a boiler ventilation flue pipe ignited "nearby combustible materials," sparking the blaze in the cellar of the building on St. Nicholas Ave. near West 149th St. in Harlem.

Fire marshals said their investigation found that the sprinkler system had been installed in a first-floor restaurant in the building, in addition to the staircase and apartments on the upper floors, but didn't activate.

The investigation also found that fire-resistant materials had been removed from the restaurant, intensifying conditions as the flames spread from the cellar.

Officials didn't confirm what the materials were, why the sprinkler system had been shut off, and didn't say if either was related to the film's production.

Firefighter Michael Davidson was overcome by smoke on March 23 as the early-morning blaze raged at the building where "Motherless Brooklyn," a movie starring Edward Norton, Bruce Willis, Willem Dafoe, and Alec Baldwin, was being filmed. Two other firefighters suffered serious burns and three civilians had minor injuries.

A tenant in the building announced a lawsuit in late-March against Norton's production company for $7 million.

Davidson was posthumously promoted to the position of lieutenant.