Someone left a burning Christmas tree in the entryway of a public housing complex at 928 Connecticut St. in the city's Potrero Hill neighborhood at about 4:30 a.m., Lt. Mindy Talmadge said.
"Someone had already extinguished it when we arrived," Talmadge said.
Talmadge said the fire burned near the front door of an apartment, and some residents were concerned there was a small child inside. Firefighters were able to enter the apartment and found that it was empty.
The fire is being investigated as a suspected case of arson. No arrests have been made.
There was minimal damage to the entryway, she said.