Fire officials had wrapped up their investigation into the origin and cause of the fire by Tuesday afternoon; that evidence will now be analyzed. They also need to speak to the two suspects in custody.
Fire officials say four homes were burning when they arrived at about 3 a.m. They were able to prevent the fire from spreading any further and everyone got out safely.
Police arrested 55-year-old Ronald Weeks when he returned to the scene. Witnesses pointed him out, telling police they had seen him running from the fire with a gas can. Police later detained another suspect, 48-year-old Erin Kennedy, who told police she is a resident of the building where the fire started.
But fire officials say it was vacant, with no electricity or gas.
Neighbors remember seeing her come out of the building, but assumed the owner, whom they identify as Jack Noonan, knew she was there.
"I was coming home and she came out the front door and closed the door, but she looked normal, and we figured Jack's nieces or nephews, you know, family members of sorts cleaning up because lately they've been cleaning up that house," neighbor Marla Racca said. "I remember her having her hair tied back in a bun...She looked like any normal citizen."
The owner is nowhere to be found.
"The police department and arson task force is trying to contact him and get some information about who had, if you could say, rights to be in the building," San Francisco Fire Department Asst. Chief Tom Siragusa said.
There was a fire in the same house three years ago to the day. Fire officials say the cause then was fireworks. The house was believed to be uninhabited since then.
Eight adults and two children were displaced by the fire. They are being cared for by family.
The cost of the damages is estimated to be around $1 million.