City arson investigators were joined by a federal team Wednesday at the scorched remains of an unfinished downtown Los Angeles apartment complex destroyed by a raging fire that also damaged adjacent office towers.

The investigation of the blaze at the huge Da Vinci complex could take weeks or months because of the complexity, officials told a press conference.

"It's a city block burned to the ground," Carlos A. Canino, special agent in charge of the ATF Los Angeles field division, said of the scope of the probe.

The 1.3 million-square-foot building was still in the wood framing stage when it ignited just before 1:30 a.m. Monday, leaving an area measuring 900 feet by 200 feet to investigate.

Suspicion of arson arose because of how quickly and completely the building became involved with flames.

"We are not treating this as a criminal investigation, but we are not ruling anything out," said Los Angeles Fire Department Chief Ralph Terrazas.

Federal assistance came in the form of a national response team from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives. They began the day with a safety evaluation of the site.

Investigators will interview witnesses, check security cameras and personal video of the fire, and will use dogs that can sniff fire accelerants.

Canino said there was a great deal of video to review, and a security guard was also being interviewed. He said the fire was initially reported to 911 by a passerby using a cellphone.

The total cost of damage has not been released. The loss of the complex was initially estimated at $10 million. Another $1.5 million in damage was done to an adjacent freeway where flames charred guardrail posts and signs. Traffic-monitoring fiber-optic cables under the pavement may have to be replaced, authorities said.

The intense heat also damaged three nearby office high-rises, blowing out or cracking hundreds of windows. Firefighters had to put out small fires in one tower, where heat also melted office equipment.

The fire set off sprinklers in offices of the city's Department of Aging, ruining 2,000 holiday gift bags containing items such as socks, sweaters and food that were going to be distributed this weekend to senior citizens, mainly the poor and those who live alone, department General Manager Laura Trejo said.

Mayor Eric Garcetti's office launched a campaign Wednesday to replace the gift bags.