The surveillance system bore witness to the last moments of 3-year-old Kyrah Martin's life.
The child was alone early Wednesday morning as she left the Crown Heights apartment she shared with her mother and siblings, as shown by footage from one camera, part of the security system in the four-story building, police said.
As her mother and three siblings slept, Kyrah climbed stairs to the roof, where a tiny set of footprints in the snow led to the ledge from which police believe she fell to her death - a "terrible tragedy," Police Commissioner Ray Kelly said.
Police responded at about 1:15 a.m. to reports of an unconscious child lying facedown in the snow with "injuries consistent with a fall."
Kyrah was rushed to Kings County Hospital Center, where she was pronounced dead at 1:40 a.m. The medical examiner will determine the cause of death.
Officers identified Kyrah by canvassing the building and locating her mother, Nefertiti Martin, 35, in a fourth-floor walk-up she had recently rented with help from the city.
Kelly said it was the second child Martin had lost: An infant born with respiratory problems died a few months after birth in 2005.
The investigation is ongoing, but Kelly said it is "probably unlikely that there will be criminal charges."
The video system showed Kyrah opening the apartment door at about 12:45 a.m., Kelly said. Alone, she walked out and through the hallway to the stairs that lead to the roof. A camera also caught her at the top of the steps, pushing open an alarmed door that leads onto the rooftop.
"The baby opens the roof door, apparently, and sets off the alarm," Kelly said. Nobody heard the ringing, however, and the alarm still was blaring when police arrived, he said.
From there, footprints on the snow-covered roof led to the ledge, Kelly said.
"It appears that it is a typical ledge," he said. "The baby looked over and fell."