The surveillance video, its images disturbingly clear, ends with a woman being burned alive in the elevator of a Brooklyn apartment building on Saturday.

But in the beginning, it seemed routine: a man dressed as an exterminator, wearing gloves, with a protective mask perched atop his head and carrying a container on his back, takes the elevator to the fifth floor.

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.