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Northbound commuter traffic into the nation's second-largest city was snarled for several hours due to the blaze, which fire officials said erupted overnight and took three hours to bring under control. No injuries were reported.
About 250 firefighters, roughly a quarter of the city's on-duty force, were battling the flames at the height of the blaze, the cause of which was under investigation, said Katherine Main, a spokeswoman for the Los Angeles City Fire Department.
Authorities were also probing a separate smaller fire that broke out across town at another construction site overnight.
Fire Captain Jamie Moore told reporters that arson investigators were treating the larger blaze "as a crime fire until proven otherwise," based in part on the size of the conflagration and the speed and intensity with which it burned.
The site that went up in flames - two stories of concrete beneath five floors of wood framing - occupied an entire city block near the junction of two major traffic arteries - the Hollywood Freeway and the Harbor Freeway.
Moments after the first alarm, firefighters whose station is located at the end of the block opened their front door to see the entire structure of the apartment project engulfed in flames, Moore said.
The fire spread quickly over 1.3 million square feet (121,000 sq meters) of space even though the exposed lumber would still have been damp from two days of heavy rains that fell late last week, he said.
Much of the structure, wrapped in scaffolding, collapsed in the flames, producing heat so intense it ignited an adjacent 16-story commercial high-rise and blew out windows of a nearby office building, raining shards of glass onto firefighters below. But crews managed to prevent that building from catching fire, Moore said.
The apartment building was a total loss, while the high-rise sustained fire damage to three floors and smoke or water damage to 12 other floors, fire officials said.
Flames spread across the Harbor Freeway at one point, prompting authorities to shut an 8-mile (13-km) stretch of the highway northbound during rush-hour traffic, along with three off-ramps into downtown from the Hollywood Freeway.
"It was just a nightmare," California Highway Patrol spokesman Edgar Figueroa said of the gridlock.