Investigators were trying to determine what sparked a high-rise fire in Manhattan early Tuesday morning that left 22 people injured.
The fire broke out around 2 a.m. on the 24th floor of a 41-story luxury building on 515 East 72nd Street on the Upper East Side.
A total of 18 people were taken to the hospital for injuries.
Officials said the fire broke out in a kitchen of one of the units and spread to the hallway. Firefighters said the resident left their front door open when they left, further fueling the flames.
"The door open is what changed this fire completely. It turned a kitchen fire — a one-room kitchen fire — into a fire that spread throughout the upper floors of the building," one FDNY official at the scene said. "Had the door been closed, this would've been one-apartment, one-room fire."
The FDNY said firefighters got the flames under control shortly before 4 a.m.
Two adults in the apartment where the fire began were listed in critical condition.
One child was treated for smoke inhalation.
Officials said four of the injured were firefighters. Their injuries were minor.
Four people refused medical attention.
Officials said the hallway was severely scorched and other damage was being assessed.
Some people, however, didn't even know a fire was raging in their building.
"We woke up and we smelled burning, so I kind of just checked the apartment and it wasn't us," one building resident said. "I feel so bad. I'm glad that it was obviously isolated, the building is structured really safely, but that's terrible that we went back to sleep."
The FDNY stressed how important it is to close doors during a fire to keep it from spreading, saying this was the second fire in the city in 24 hours in which a door was left open, resulting in injuries.
Officials say the building is now safe for residents.