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 told The San Francisco Chronicle that authorities do not know 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.
"''We just can't tell," she said. "It is a very difficult scene. The medical examiner is going to sort out for us the cause of death."
The victims' identities have yet to be released.
Investigators arrived back on the scene Saturday, and continued to probe how the victims died.
The neighborhood in the southern part of the city is home to Lick-Wilmerding High School and San Francisco's City College, in addition to 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."
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, said Police Chief Greg Suhr. 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.
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Information from: San Francisco Chronicle, http://www.sfgate.com
