Sometime later, an older woman carrying groceries took the same ride to the fifth floor.       
Two cameras recording from different positions, one inside the small tiled elevator and another in a hallway, show the doors open and the man with the container approach. The man, who appeared to be in his 40s, first sprays the woman in the face, then douses her methodically from head to toe with what a city official said was an accelerant as she turned and cowered, raising her hands, the grocery bags hanging from her wrists.       
Having cornered the woman in the elevator, the man struggles to light a barbecue lighter. He then ignites a Molotov cocktail — a wine or Champagne bottle filled with accelerant with a rag stuffed in its neck. He retreats and comes back again, spraying more liquid on his victim. And suddenly the silent video goes white with a conflagration in the small space: the woman, on fire.        
Investigators are poring over the footage, a disturbing silent film capturing what is perhaps a singular act of violence: a woman being burned to death.       
The crime took place Saturday afternoon at 203 Underhill Avenue in Prospect Heights. Detectives and fire marshals were reviewing the footage and interviewing neighbors.       
The police identified the victim as Doris Gillespie, 64, and pronounced her dead at the scene. One neighbor said that Ms. Gillespie was a postal worker.