Police initially were investigating the deaths of three women and two men in the Ingleside District as a possible murder-suicide.
But police spokeswoman Cmdr. Lyn Tomioka said during a press briefing Saturday afternoon that authorities have not determined whether the body of the killer or killers is inside the home, nor do they know what weapons were used.
Tomioka said the crime scene was so tangled it was hard to assess what exactly had happened.
"It could take days to process," Tomioka said. "We are working around the clock to determine what happened here."
Police had not determined a motive, had no suspect information and could not say when any names would be released, but Tomioka said members of the public should not consider themselves at risk because of the killings.
"It is specific to this address and only this address, so there is no threat to public safety," she said.
Yellow police tape sealed off the entire block, as investigators dressed in white hazmat suits walked in and out of the orange row house carrying plastic bags. Police would not say what was in the bags, and reporters were kept at a distance.
A makeshift shrine outside the house grew as more people dropped bouquets.
The neighborhood, located in the southern part of the city, is home to Lick-Wilmerding High School and San Francisco's City College, and has a thriving immigrant community, largely from Asia.
"It's an active, ongoing investigation," police Sgt. Daryl Fong said. "They're looking at all information and all types of evidence and still looking to determine what happened."
Neighbors appeared shaken by the killings.
"It's calm and peaceful here usually, but nowadays neighbors don't know each other so anything can go on," said Jovi Geaga, 26, a resident and a tutor at a nearby church.
Neighbor Miriam Mendoza Moody said she heard a "loud male person angry or yelling at around midnight" on Thursday, but said she did not hear gunshots.
A woman with access to the home discovered the bodies early Friday morning.
She found one man shot to death in the foyer by the front door, police Chief Greg Suhr said. She then saw a man and a woman dead in the garage before running out and calling police.
When officers arrived, they found the other two deceased women and determined that at least two of the victims were shot.
Mayor Ed Lee called the incident a terrible tragedy.
"As the San Francisco Police Department thoroughly investigates this incident, I extend, on behalf of the city, our support and sympathy to all family members and friends of the victims involved in this crime," Lee said in a statement.