The massive fire that consumed a downtown Los Angeles development under construction came at the most vulnerable moment.
Sprinklers were not yet installed in the sprawling seven-story Da Vinci apartment complex. Walls had not yet been put in, making oxygen readily available to fuel the blaze as it rose to infernal heights. The building had not been "compartmentalized," a standard fire protection measure designed to prevent flames from spreading from one part of the building to the next.
Los Angeles Fire Department officials have described the fire — which also badly damaged two nearby office towers and caused tens of millions of dollars in damage — as suspicious. But it will probably take days or weeks for investigators to fully determine whether the blaze was deliberately set.
A federal team of arson investigators with the Bureau of Alcohol, Tobacco, Firearms and Explosives arrived in Los Angeles on Tuesday to begin its investigation Wednesday.
Certain aspects of the fire suggest that arson is a possibility, experts said.
The blaze began in the middle of the night, making it less probable that it was accidentally caused by a construction worker's equipment or a carelessly discarded cigarette. It came at a stage during the building's construction when none of the fire protections had been put in, with exposed wood frames ideal for a quick-spreading fire.
"You'd expect, if it's an accident, to occur during working hours," said Edward Nordskog, a veteran arson investigator with the Los Angeles County Sheriff's Department who is not involved in the current inquiry. "If you look at the situation and you see a fire at midnight or at 1 a.m., that's suspicious on its face right there."
Experts cautioned that because the fire burned through the development so thoroughly — only scattered stairwells and skeletal remains of wooden frames on the northern end of the building remained standing — determining the origin of the fire will be difficult, if not impossible.
"It looked pretty hopeless actually, in being able to pinpoint a cause unless someone did something stupid and very obvious," said John Lentini, a Florida-based consultant and a nationally recognized arson expert.
Lentini said that because of the extent of the fire, surveillance footage from nearby businesses or video captured by passerby on their smartphones may prove critical.
Fire investigators will start digging through the debris by hand Wednesday, said Dan Heenan, leader of the 20-agent ATF national response team that was called in to assist the investigation. Pieces of rubble showing a burn pattern or other indications of the fire's progression will be preserved and documented and the rest will be carted away, he said. An accelerant-detecting canine will also be sniffing through the site, and chemists, fire protection engineers and electrical engineers will be on hand to provide their expertise.
At the same time, investigators will interview initial responders and any witnesses, and look for video footage that may show where the fire started.
Heenan said that rather than start with a hypothesis about how the fire may have ignited, investigators will begin by looking at any and all possibilities and rule them out one by one as the inquiry progresses.
"If it turns out to be incendiary, would we look at eco-terrorists? Absolutely. Homeless people? Absolutely," he said. "Everything's open right now."
At the end of the investigation, the response team will reach one of three conclusions, Heenan said: The fire was intentionally set, it was accidentally set or the cause is undetermined. In about 20% of its cases, the team has concluded that the cause could not be determined, he said.
The response team's investigation usually takes about a week, he said, but the downtown blaze is one of its larger cases.
"I told my wife I'm thinking 14 days," said Heenan, who is based in Las Vegas. "I'm hoping it's not 17, because that's Christmas and my wife will be upset."
Meanwhile, workers remained displaced from two city-owned buildings near the development that were also damaged in the fire. City officials said employees will be able to return to one of the buildings by Thursday, but the other, which houses workers for the Departments of Aging, Recreation and Parks, and Animal Services, will remain closed for an extended period.
Times staff writers Soumya Karlamangla, Joseph Serna and Ruben Vives contributed to this report.