Los Angeles fire officials said they are "inclined" to believe a fire that engulfed a massive residential development project downtown was intentionally set.
But until arson investigators can enter the wreckage, it's impossible to determine the cause, which could take several days.
"Certainly one of the things we lean toward is 'was it intentionally set?'" LAFD Deputy Chief Joseph Castro said at a news conference Monday afternoon.
Investigators will collect samples, review video surveillance and canvass the site to determine the cause of the fire and find the origin. They have yet to enter the building because it's structurally unsafe and its charred skeleton was still too hot.
John D'Angelo, the assistant special agent in charge of the Bureau of Alcohol, Tobacco, Firearms and Explosives in Los Angeles, said ATF agents are helping with the investigation as part of a Southern California task force.
Several detectives from the LAPD's Major Crimes division were working with arson investigators "to assist with whatever the Fire Department needs," said LAPD Cmdr. Andrew Smith.
"It's too soon to speculate what caused this fire," said Los Angeles City Councilman Jose Huizar, who represents the downtown area.
The fire started about 1:20 a.m. in a complex the size of a city block, off Figueroa Street straddling Temple Street, and quickly consumed the building's wood framing.
It was a "remarkable firefight," Castro said. The first firefighters on scene found flames stretching 700 feet down Figueroa climbing more than seven stories into the air.
The flames climbed so high and the heat was so powerful that it blew out the windows on 16 stories of a neighboring building and started small fires on seven of them.
More than 200 firefighters rushed to the scene -- a response in size and speed that Castro said was "unheard of throughout the nation."
Firefighters had to work under intense heat and avoid falling shards of glass.
Though no one was injured in the blaze, the financial toll is widespread.
A California Department of Transportation official said an exit sign over the 110 Freeway melted and the framing would have to be replaced -- forcing another freeway closure later this week.
The department's fiber optic lines below the road that monitor traffic flow and speeds may also have to be replaced, Caltrans spokesman Patrick Chandler said.
At the same time, Los Angeles' parks and recreation and health departments were forced out of their buildings because of collateral damage, officials said.
Fire officials said portions of the apartment building left standing will be demolished in the upcoming days.
Anyone with information about the fire and how it started is urged to call a hotline at (213) 893-9850.