The Chinatown fire that killed a 91-year-old woman and her 52-year-old son was accidental and caused by extension cords, FDNY marshals said Saturday.

The six-story building on Mulberry Street was deemed structurally sound and not in imminent danger of collapse by the city's Department of Buildings, but all residents were ordered to vacate. The American Red Cross is working to help tenants relocate.

A smoke alarm was "present and operational," the FDNY said after buildings inspectors completed their investigation.

FDNY crews were called to the scene just after 3:45 a.m. Friday.

Authorities said the fire was located on the fifth floor, and had spread into the hallway and public areas of the building. DOB investigators reported part of the sixth floor stairwell collapsed onto the fifth floor.

The blaze quickly rose to two alarms with more than 100 fire personnel responding to the scene.

Four others, including two firefighters, suffered minor injuries, authorities said.

On Saturday, the FDNY's Fire Safety Education Unit was outside the building sharing safety information with community members