A fire that reduced an unfinished downtown apartment building to a block-long mountain of blackened wood was an act of arson that caused an estimated $20 million to $30 million in damage, the Fire Department announced Thursday.

The Dec. 8 fire burned the seven-story, 1.3-million-square-foot Da Vinci complex.

Investigators from the Fire Department and a national response team from the Bureau of Alcohol, Tobacco, Firearms and Explosives sifted through 75,000 square feet of debris to find the cause, a Fire Department statement said.

Investigators "recovered sufficient evidence to eliminate all known potential accidental causes and determine the fire was intentionally set," the statement said. Potential evidence will be submitted to the ATF's national laboratory for analysis.

Details weren't being released because the investigation continues, but authorities are seeking two people who were seen in video near the scene the morning of the fire.

The men aren't considered suspects or persons of interest, but investigators want to interview them, the Fire Department said.

Heat from the blaze was so intense that it melted a sign on a nearby freeway and cracked or shattered hundreds of windows in office buildings, including the city Department of Water and Power headquarters a block away.

The fire set off sprinklers in offices of the city's Department of Aging, ruining 2,000 holiday gift bags that contained items such as socks, toiletries and blankets to be distributed to senior citizens.

However, a fundraising drive begun by Amy Elaine Wakeland, the wife of Mayor Eric Garcetti, raised enough money to not only replace the packages but to pay for gifts for all 4,500 seniors who receive meals delivered to their homes under a Department of Aging program.