Seven people were taken to hospitals, and four were treated and released, fire Capt. Erik Scott said, adding that the two most seriously injured were 30-year-old men. Two firefighters suffered minor burn injuries, he said.
Fifteen people, including injured residents, were rescued by helicopter from the rooftop.
"It's a valuable resource for our helicopter to do rooftop evacuations," Fire Chief Ralph Terrazas said. "It was very successful."
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The fire started on the sixth floor and damaged units on the fifth, seventh and eighth levels before 335 firefighters put it out, officials said. Arson investigators were at the scene.
"It's very early on with the investigation," Los Angeles police Capt. Randall Goddard said at a news conference Wednesday evening. "We're still trying to determine was this fire intentionally set, or was it not intentionally set, was it an accident, and we don't have those answers yet."
When firefighters arrived, they found a "well-entrenched" fire on the seventh floor fueled by wind gusts of 35 mph, Scott Said.
"We were met by residents that were literally hanging out of windows," Scott said.
Intense flames injured the two firefighters.
"These firefighters were literally driven to their bellies halfway through that hallway before they even got to that fire unit," he said.
Sierra Menni, 28, who lives across the street, said she heard loud bangs before seeing flames and a man dangling out a window.
"I heard two explosions and people screaming, and I ran out, and I went to the front of my building, and I looked up and saw huge flames, and there was a guy with his foot out the window," Menni said in an interview.
"He was just sitting on the window," Menni said. "Practically with one leg out of the window. Literally, the window next to the other window that was on fire."
A fire truck's ladder got to the man, and he was lowered to safety.
"They got him down, and I saw him wheel-chaired away," Menni said. "He seemed kind of ashy but awake and coherent and OK."
She added: "The firefighters were honestly amazing."
All 339 occupants of the 240 units were evacuated, Scott said, adding that officials deemed the building unsafe for habitation but that inspectors would re-evaluate Thursday.
The building, which was last inspected in June, was up to code at the time of the fire, Scott said. It was not required to have sprinklers because it was built before 1974. It is among 55 residential high-rises in the city that do not have sprinklers, he said.
Andrew Blankstein reported from Los Angeles and Shamar Walters and David K. Li from New York.
Andrew Blankstein is an investigative reporter for NBC News. He covers the Western United States, specializing in crime, courts and homeland security.
Shamar Walters is a reporter for NBC News' Social Newsgathering team and is based in New York City.
David K. Li
David K. Li is a breaking news reporter for NBC News.